Introduction to Data Communications

Data Communications

Data Communications is the transfer of data or information between a source and a receiver. The source transmits the data and the receiver receives it. The actual generation of the information is not part of Data Communications nor is the resulting action of the information at the receiver. Data Communication is interested in the transfer of data, the method of transfer and the preservation of the data during the transfer process.

In Local Area Networks, we are interested in "connectivity", connecting computers together to share resources. Even though the computers can have different disk operating systems, languages, cabling and locations, they still can communicate to one another and share resources.

The purpose of Data Communications is to provide the rules and regulations that allow computers with different disk operating systems, languages, cabling and locations to share resources. The rules and regulations are called protocols and standards in Data Communications.

5. Why Telecommunications?

What does networking have to do with telephones? Telephones and networking work hand in hand. The telecommunications industry has been gradually integrating with the computer industry and the computer industry has been gradually integrating with the telecommunications industry. The common goal is to join distantly located Local Area Networks into Metropolitan and Wide Area Networks (MANs and WANs).

5a. Voice Channels

First thing that comes to mind is telephone systems and the phone at home. Talking to someone on the phone uses Voice Channels. This doesn't seem to have much to do with Networks!

We do use voice channels for modem communications to connect to BBSs (Bulletin Board Services) or to connect to the Internet. We also use voice channels to connect LANs using remote access. Due to the bandwidth limits on the Voice Channel, the data transfer rate is relatively slow.

Voice Channel: Dial-up connection through a modem using standard telephone lines. Typical Voice Channel communication rates are: 300, 1200, 2400, 9600, 14.4k, 19.2k, 28.8k, 33.6k and 56 kbps (bits per second).

5b. Data Channels

Data channels are dedicated lines for communicating digitized voice and data. At the end of 1996, there was a major milestone where more data was communicated in North America's telecommunications system than voice. 

5b. Data Channels (cont'd)

Data Channels are special communications channels provided by the "common carriers" such as Telus, Sprint, Bell Canada, AT&T, etc.. for transferring digital data. Data Channels are also called "Leased Lines". They are "directly" connected and you don't have to dial a connection number. The connections are up and running 24 hours per day. They appear as if there were a wire running directly between the source and destination. Typical transfer rates for data communication are: 56 k, 128k, 1.544 M, 2.08 M, 45M and 155 Mbps.

Common carriers charge for data connections by

1. the amount of data transferred (megabytes per month)2. the transfer rate (bits per second)

3. the amount of use (time per month)

6. Introduction to NetworkingWhat is a Network? This is a difficult question to answer. A network can consist of two computers connected together on a desk or it can consist of many Local Area Networks (LANs) connected together to form a Wide Area Network (WAN) across a continent.

The key is that 2 or more computers are connected together by a medium and they are sharing resources. The resources can be files, printers, hard drives or cpu number crunching power.

6a. The Big Picture

Many individuals have asked to see The Big Picture of networking: "where does everything fit in?". Where does Microsoft NT fit in with routers and the OSI layers?

What about UNIX, Linux and Novell? The following page has a graphic showing The Big Picture. It attempts to show all areas of networking and how they tie into each other. The following key describes the graphical symbols used:

Circles  Network Operating Systems Squares  Communication & cabling protocols (OSI Transport to Physical

Layer)

Storm Clouds  Telecommunications media or Information providers that connect to the Internet

Machine symbol  Network "linker" can be a Bridge, Router, Brouter or Gateway

The Internet  jagged haphazard dotted line

6b. Telecommunications Components of The Big Picture

ISDN  Integrated Services Digital Network Private Branch Exchanges  PBXs, Key Systems

Telcos  AT&T, Bell Telephone, Sprint, Telus

DataPac & DataRoute  packet switching and analog switching WAN protocols

Cell Relay  Digital packet switching WAN protocol

Frame Relay  Digital packet switching WAN protocol

X.25  Analog packet switching WAN protocol

ATM  Asynchronous Transfer Mode WAN protocol

World Wide Web  Hypertext based multimedia system

ADSL  Asymmetrical digital subscriber line

6c. ISO OSI

The International Standards Organization (ISO) Open Systems Interconnect (OSI) is a standard set of rules describing the transfer of data between each layer. Each layer has a specific function. For example the Physical layer deals with the electrical and cable specifications.

The OSI Model clearly defines the interfaces between each layer. This allows different network operating systems and protocols to work together by having each manufacturere adhere to the standard interfaces. The application of the ISO OSI model has allowed the modern multi protocol networks that exist today. There are 7 Layers of the OSI model:

7. Application Layer (Top Layer) 6. Presentation Layer

5. Session Layer

4. Transport Layer

3. Network Layer

2. Data Link Layer

1. Physical Layer (Bottom Layer)

The OSI model provides the basic rules that allow multiprotocol networks to operate. Understanding the OSI model is instrument in understanding how the many different protocols fit into the networking jigsaw puzzle. The OSI model is discussed in detail in Introduction to the ISO - OSI Model.

7. Breaking The Big Picture up!The Big Picture still doesn't give us a good idea of the placement of the many protocols involved in networking and telecommunications. The Big Picture can be broken up according to their protocols into the following 4 areas:

7a. Local Loop  , 7b. LANs  ,

7c. MANs  and

7d. WANs.

7a. The Local Loop

The Local Loop is often called "the last mile" and it refers to the last mile of analog phone line that goes from the central office (CO) to your house. Typical local loop protocols are:

Voice lines Modem connections  56 kbps

ISDN (Integrated Services Digital Network)  2 x 64 kbps digital lines

ADSL (Asymmetrical Digital Subscriber Line)  up to 8 Mbps

Cable Modems  up to 30 Mbps

Note: Cable modems are not part of the Local Loop but do fall in the category of "the last mile" or how to get high speed digital communication to the premise (home). It would incredibly expensive to replace the existing cabling structure. All of these protocols are used to overcome the existing cabling limitations in the local loop and provide high speed digital data tranmission. The existing cabling was designed for voice communications and not digital.

7b. LANs

LANs (local area networks) are networks that connect computers and resources together in a building or buildings close together.

The components used by LANs can be divided into cabling standards, hardware and protocols. Examples of cabling standards used on LANs are:

Cat 3, 4 and 5 cables IBM Type 1 9 cabling standards

EIA568A and 568B

Ethernet cabling standards: IEEE 802.3 (10Base5), IEEE 802.3a (10Base2), IEEE 802.3i (10BaseT)

Unshielded Twisted Pair (UTP)

Shielded Twisted Pair (STP)

Connectors: RJ45, RJ11, Hermaphroditic connectors, RS 232, DB 25, BNC, TEE

b. LANs (cont'd)

Examples of hardware devices are: Network Interface Cards NICs Repeaters

Ethernet Hubs or multiport repeaters

Token Ring MultiStation Access Units (MSAUs), Control Access Units (CAUs) and Lobe Access Modules (LAMs)

Bridges

Brouters

Routers

Gateways

Print servers

File servers

Switches

Examples of LAN protocols are:

Ethernet frame types: Ethernet_II, Ethernet_SNAP, Ethernet_802.2, Ethernet_802.3

Media Access Control layer (MAC layer)

Token Ring: IBM and IEEE 802.5

Logical Link Control Layer (LLC) IEEE 802.2

TCP/IP

SMB, NetBIOS and NetBeui

IPX/SPX

Fiber Distributed Data Interchange (FDDI)

Asynchronous Transfer Mode (ATM)

7c. MANs

Metropolitan Area Networks (MANs) are networks that connect LANs together within a city.

The main criteria for a MAN is that the connection between LANs is through a local exchange carrier (the local phone company). The protocols that are used for MANs are quite different from LANs except for ATM which can be used for both under certain conditions.

Examples of MAN protocols are:

RS 232, V 35 X.25 (56kbps), PADs

Frame Relay (up to 45 Mbps), FRADs

Asynchronous Transfer Mode (ATM)

ISDN (Integrated Services Digital Network) PRI and BRI

Dedicated T 1 lines (1.544 Mbps) and Fractional T 1

T 3 (45 Mbps) and OC 3 lines (155 Mbps)

ADSL (Asymmetrical Digital Subscriber Line)  up to 8 Mbps

xDSL (many different types of Digital Subscriber Lines)

7d. WAN Wide Area Networks (WANs) connect LANs together between cities.

The main difference between a MAN and a WAN is that the WAN uses Long Distance Carriers. Otherwise the same protocols and equipment are used as a MAN.

8. Trade MagazinesIn 1994, TCP/IP was considered dead by many  Unix was considered obsolete. World Wide Web didn't exist as we know it today! Today TCP/IP is the king of network transport protocols! In a matter of months, the computing world completed reversed its direction. The only way to keep current in the computing industry is to read trade publications.

Educational institutes are not able to keep up with the pace of the computing industry. The fast track education cycle takes 6 months to a year to propose, develope and finally run a new course! In that time, there could be major changes or revisions of the product. An excellent example of change is the Linux kernel revisions over the past year.

Anything you read that is over 2 years old is pretty much obsolete! For example: anything you read about fibre optics that is 3 months old is obsolete. To succeed you must read regularly every trade and computer magazine possible. You just have to skim the magazines and read only the articles that are of interest.

There are many free trade publications available to the computing industry if you qualify. Some examples are:

Free Publications:

Internetwork Computing Canada

Comnputer Service News

Communication News

LAN Computing

The Computer Paper

Other publications that are worthwhile reading are:

Byte Magazine MacWorld

PC Computing

Linux Journal

LAN magazine

Most trade magazines now offer webpage versions of their magazines on the Internet. In addition, they provide a searchable database of previous articles and programs. Access to the Internet is a necessity if you are going to succeed in the field of network computing. Examples of online resources are:

Linux Gazette Slashdot (news for nerds)

ZDnet

Linux Documentation Project

Linux.org

9. The Role of Telecommunications in Networking

From The Big Picture, we see that telecommunications provides a connection service (storm clouds) between networks (circles). Telecommunications provides the external connection service for joining networks across cities, provinces and countries.

9a. LANs Local Area Networks - a system of computers that share resources such as

hard-drives, printers, data, CPU power, fax/modem, applications, etc... They

usually have distributed processing - means that there is many desktop computers distributed around the network and that there is no central processor machine (mainframe). Can be campus wide like a college or university.

Location: In a building or individual rooms or floors of buildings or nearby buildings.

9b. MANs Metropolitan Area Networks: a system of LANs connected through out a city

or metropolitan. MANs are used to connect to other LANs. A MAN has to have the requirement of using a telecommunication media such as Voice Channels or Data Channels. Branch offices are connected to head offices through MANs. Examples of companies that use MANs are universities and colleges, grocery chains and banks.

 Location: Separate buildings distributed throughout a city.

9c. WANs Wide Area Networks: a network system connecting cities, countries, continents

together. TransCanada Pipeline has a WAN that stretches from Alberta to Boston. It goes from Alberta to Ontario then through the States and ends up in

Boston. The maintenance and control of the network resides in Calgary. WANs are connected together using one of the telecommunications media.

Location: City to city, across a country or across a continent.

10. Brief History of NetworkingThe following is a brief history of computers, networking and telecommunication milestones:

1. CRT (Cathode Ray Tube) credited to Braun in 1897

2. Teletype (telegraph 5 bit) during WW1

3. ARQ (Automatic Repeat reQuest) credited to Van Duuren during WWII

error checking and auto request for retransmission

4. ENIAC credited to DOD / MIT during WWII

Electronic Numerical Integrator And Calculator Used for decoding enemy messages

1st generation computer: used vacuum tubes

Programmed with jumpers and switches

MTBF (Mean Time Between Failure): 7 minutes

337 multiplications per second

5. SAGE (Semi-Automatic Ground Environment) MIT 1950s

23 centres for ground/air enemy detection systems error checking, keyboard & CRT terminals

duplexed computers, voice grade (300-4KHz)

300 baud, light pens, multiuser system

magnetic core memory

Ground to air data Tx

1st commercial use was Sabre Reservation System

6. Jacquard's Loom

First programmable machine

7. Transistorized Computers - 2nd Generation 1960s

One of the 1st inventors: Cray Batch programming: 1 pgm @ a time

Punch cards

Stored programs: held in memory

50K instructions/second

ex. IBM 7905

8. CTSS (Compatible Time Sharing System) credited to Cobato/MIT in 1961

time slices multiusers

9. Synchronous Orbit Communication Satellites. Idea by Arthur C. Clarke in 1945

Geostationary orbit around equator by Rose/Hughes Aerospace in1963

36,000 miles altitude

10. LASER credited to Maiman in 1960

A narrow band source of optical radiation suitable for use as a carrier of info.

Light Amplification by Stimulated Emission of Radiation

11. T-1 Carrier System credited to Bell Labs in 1961

TDM (Time Domain Multiplexing) 24 channels = 64 Kbps ea.

1.544 Mbps (mega bits per sec)

12. RS232 developed in 1960 and revised since.

Standard plug and "protocol" convention between modems and machines: 25 pin

Europe uses V.24 compatible standard

13. Auto Equalization Techniques of Phone lines credited to Lucky et al. in 1965

adapt to characteristics of telephone line to increase speed

14. Fibre Glass credited to Kao & Hockman in 1966

proposed "fibre glass " optics developed at Standard Telecom Labs

15. Integrated Circuits Computers - 3rd Generation - 1967

SSI/MSI (Small Scale Integration/Medium Scale Integration) 10 transistors/chip and 100 transistors/chip

Multi-user systems

Multitasking

16. Carterfone - FCC Decision in 1968 -

FCC decision allows other manufacturer's to use phone lines

opens up competition among phone systems

17. Low-loss Fibre credited to Kapron in 1970

speeds: 45-90 Mbps developed at Corning Glass Works 1984: attained 405-565 Mbps in single mode

Early 1990s: attained 1.7 Gbps

18. ARPA Network (ARPANET) developed by the DOD in the 1970s

Advanced Research Projects Agency of the Department of Defence - US 1st use of Packet Switching, layered protocols

Beginning of the Internet

19. VLSI Integration - 4th Generation Computers developed by Intel in 1971

Very large scale integration: 20,000+ transistors/chip Intel 4004 microprocessor - 4 bit

Grandparent of processors today

20. Layered Network Architecture

SNA: System Network Architecture IBM Mainframe

DNA: Digital Network Architecture DEC for DECNET

21. Ethernet developed by Xerox in 1974 -

Ether is the mysterious invisible fluid that transfers heat

Originally based on the ALOHA radio protocol

22. Videotex developed by Teletel (France) in the 1980s

Interactive video Minitel

23. Reference Model for Open Systems Interconnect developed by the ISO in 1983

Continuously evolving model for layering network protocols

24. AT&T Divestiture in 1984 -

Break-up of AT&T monopoly into Baby Bells

25. ISDN developed in 1984 -

Integrated Services Digital Network Strong in Europe

A network evolving from a telephony integrated digital network supporting: voice, teletex, videotex, fax, slowscan video, etc..

26. Linux Version 0.01 released Sept 17, 1991

11. Data Communication NetworkThe major criteria that a Data Communication Network must meet are:

i. 11a. Performance ii. 11b. Consistency

iii. 11c. Reliability ,

iv. 11d. Recovery  and

v. 11e. Security

11a. Performance

Performance is the defined as the rate of transferring error free data. It is measured by the Response Time. Response Time is the elasped time between the end of an inquiry and the beginning of a response. Request a file transfer and start the file transfer. Factors that affect Response Time are:

a. Number of Users: More users on a network - slower the network will runb. Transmission Speed: speed that data will be transmitted measured in bits per

second (bps)

c. Media Type: Type of physical connection used to connect nodes together

d. Hardware Type: Slow computers such as XT or fast such as Pentiums

e. Software Program: How well is the network operating system (NOS) written

11b. Consistency

Consistency is the predictability of response time and accuracy of data.a. Users prefer to have consistent response times, they develop a feel for normal

operating conditions. For example: if the "normal" response time is 3 sec. for printing to a Network Printer and a response time of over 30 sec happens, we know that there is a problem in the system!

b. Accuracy of Data determines if the network is reliable! If a system loses data, then the users will not have confidence in the information and will often not use the system.

11c. Reliability

Reliability is the measure of how often a network is useable. MTBF (Mean Time Between Failures) is a measure of the average time a component is expected to operate between failures. Normally provided by the manufacturer. A network failure can be: hardware, data carrying medium and Network Operating System.

11d. Recovery

Recovery is the Network's ability to return to a prescribed level of operation after a network failure. This level is where the amount of lost data is nonexistent or at a minimum. Recovery is based on having Back-up Files.

11e. Security

Security is the protection of Hardware, Software and Data from unauthorized access. Restricted physical access to computers, password protection, limiting user privileges and data encryption are common security methods. Anti-Virus monitoring programs to defend against computer viruses are a security measure.

11f. Applications

The following lists general applications of a data communication network:i. Electronic Mail (e-mail or Email) replaces snail mail. E-mail is the forwarding

of electronic files to an electronic post office for the recipient to pick up.

ii. Scheduling Programs allow people across the network to schedule appointments directly by calling up their fellow worker's schedule and selecting a time!

iii. Videotext is the capability of having a 2 way transmission of picture and sound. Games like Doom, Hearts, distance education lectures, etc..

iv. Groupware is the latest network application, it allows user groups to share documents, schedules databases, etc.. ex. Lotus Notes.

v. Teleconferencing allows people in different regions to "attend" meetings using telephone lines.

vi. Telecommuting allows employees to perform office work at home by "Remote Access" to the network.

vii. Automated Banking Machines allow banking transactions to be performed everywhere: at grocery stores, Drive-in machines etc..

viii. Information Service Providers: provide connections to the Internet and other information services. Examples are Compuserve, Genie, Prodigy, America On-Line (AOL), etc...

ix. Electronic Bulletin Boards (BBS - Bulletin Board Services) are dialup connections (use a modem and phone lines) that offer a range of services for a fee.

x. Value Added Networks are common carriers such as AGT, Bell Canada, etc.. (can be private or public companies) who provide additional leased line connections to their customers. These can be Frame Relay, ATM (Asynchronous Transfer Mode), X.25, etc.. The leased line is the Value Added Network.

11g. Basic Components

Source: It is the transmitter of data. Examples are:

Terminal, Computer,

Mainframe

Medium: The communications stream through which the data is being transmitted. Examples are:

Cabling, Microwave,

Fibre optics,

Radio Frequencies (RF),

Infrared Wireless

Receiver: The receiver of the data transmitted. Examples are:

Printer, Terminal,

Mainframe,

Computer,

DCE: The interface between the Source & the Medium, and the Medium & the Receiver is called the DCE (Data Communication Equipment) and is a physical piece of equipment.

DTE: Data Terminal Equipment is the Telecommunication name given to the Source and Receiver's equipment.

An example of this would be your PC dialing into a BBS (Bulletin Board System):

12. Data FlowData flow is the flow of data between 2 points. The direction of the data flow can be described as:

Simplex: data flows in only one direction on the data communication line (medium). Examples are Radio and Television broadcasts. They go from the TV station to your home television.

Half-Duplex: data flows in both directions but only one direction at a time on the data communication line. Ex. Conversation on walkie-talkies is a half-duplex data flow. Each person takes turns talking. If both talk at once - nothing occurs!

Bi-directional but only 1 direction @ a time!

HALF-DUPLEX

Full-Duplex: data flows in both directions simultaneously. Modems are configured to flow data in both directions.

Bi-directional both directions simultaneously!

FULL-DUPLEX

13. ModemsA modem is a Modulator/Demodulator, it connects a terminal/computer (DTE) to the Voice Channel (dial-up line).

13a. Basic Definition

The modem (DCE - Data Communication Equipment) is connected between the terminal/computer (DTE - Data Terminal Equipment) and the phone line (Voice Channel). A modem converts the DTE (Data Terminal Equipment) digital signal to an analog signal that the Voice Channel can use.

A modem is connected to the terminal/computer's RS232 serial port (25 pin male D connector) and the outgoing phone line with an RJ11 cable connector (same as on a phone extension cord). Male connectors have pins, female connectors have sockets.

13b. Digital Connection

The connection between the modem and terminal/computer is a digital connection. A basic connection consists of a Transmit Data (TXD) line, a Receive Data (RXD) line and many hardware hand-shaking control lines.

The control lines determine: whose turn it is to talk (modem or terminal), if the terminal/computer is turned on, if the modem is turned on, if there is a connection to another modem, etc..

13c. Analog Connection

The connection between the modem and outside world (phone line) is an analog connection. The Voice Channel has a bandwidth of 0-4 kHz but only 300 - 3400 Hz is usable for data communications.

The modem converts the digital information into tones (frequencies) for transmitting through the phone lines. The tones are in the 300-3400 Hz Voice Band.

13d. External/Internal Modems

There are 2 basic physical types of modems: Internal & External modems. External modems sit next to the computer and connect to the serial port using a straight through serial cable.

Internal modems are a plug-in circuit board that sits inside the computer. It incorporates the serial port on-board. They are less expensive than external modems because they do not require a case, power supply and serial cable. They appear to the communication programs as if they were an external modem for all intensive purposes.

13e. Modem Types

There are many types of modems, the most common are:i. Optical Modems

Uses optical fibre cable instead of wire. The modem converts the digital signal to pulses of light to be transmitted over optical lines. (more commonly called a media adapter or transceiver)

ii. Short Haul ModemsModems used to transmit over 20 miles or less. Modems we use at home or to connect computers together between different offices in the same building.

iii. Acoustic ModemA modem that coupled to the telephone handset with what looked like suction cups that contained a speaker and microphone. Used for connecting to hotel phones for travelling salespeople.

iv. Smart ModemModem with a CPU (microprocessor) on board that uses the Hayes AT command set. This allows auto-answer & dial capability rather than manually dialing & answering.

v. Digital ModemsConverts the RS-232 digital signals to digital signals more suitable for transmission. (also called a media adapter or transceiver)

vi. V.32 ModemMilestone modem that used a 2400 Baud modem with 4 bit encoding. This results in a 9600 bps (bits per second) transfer rate. It brought the price of high speed modems below $5,000.

Baud is the speed at which the Analog data is changing on the Voice Channel and bps is the speed that the decoded digital data is being transferred.

13f. Features of Modems

1. SpeedThe speed at which the modem can send data in bps (bits per second). Typically modem speeds are: 300, 600, 1200, 2400, 4800, 9600, 14.4K, 19.2K, 28.8K bps

2. Auto Dial /RedialSmart Modems can dial the phone number and & auto redial if a busy signal is received.

3. Auto AnswerMost modems can automatically answer the phone when an incoming call comes in. They have Ring Detect capability.

4. Self-TestingNew modems have self-testing features. They can test the digital connection to the terminal /computer and the analog connection to a remote modem. They can also check the modem's internal electronics.

5. Voice over DataVoice over Data modems allow a voice conversation to take place while data is being transmitted. This requires both the source and destination modems to have this feature.

6. Synchronous or Asynchronous TransmissionNewer modems allow a choice of synchronous or asynchronous transmission of data. Normally, modem transmission is asynchronous. We send individual characters with just start and stop bits. Synchronous transmission or packet transmission is used in specific applications.

7. 3g. Modem Speeds / Standards

Bell 103 300 bps FSK -Half duplex

Bell 113 300 bps FSK - Full duplex

Bell 202 1200 baud half duplex

Bell 212A

1200 bps DPSK (Dibit Phase Shift Keying) - V.22 compatible300 bps FSK (Frequency Shift Keying) - NOT V.22 compatible

MNP1-3 Microcon Networking Protocol - Basic error detection and control of errors.

MNP4 Error correction + adapts to line conditions.

MNP5 Error correction + adapts to line conditions and adds Compression technique used to double the data transfer rate.

RS-232D Cable and connector standard

V.22 1200 bps DPSK (Dibit Phase Shift Keying) - Bell 212A compatible600 bps PSK (Phase Shift Keying) - NOT Bell 212A compatible

V.22bis2400 bps - International StandardFallback in Europe to V.22Fallback in America to Bell 212A

V.24 European Mechanical specifications for RS-232D

V.26 . Synchronous 2400 bps modem1200 bps DPSK full duplex

V.27 Synchronous 4800 bps DPSK modem

V.28 European Electrical specifications for RS-232D

V.29 Synchronous 9600 bps QAM

V.32 9600 bps QAM

V.32bis 14.4 Kbs QAM1

V.33 14.4 Kbps Trellis Coded Modulation for noise immunity.

V.34 28.8 Kbps modem standard

V.34bis 33.6 Kbps modem standard

V.42bis Compression technique to roughly double the data transfer rate. Uses Automatic Repeat Request ARQ and CRC (Cyclic Redundancy Checking)

WE201 Synchronous Western Electric 2400 bps DPSK

WE208 Synchronous 4800 bps DPSK

WE209 Synchronous 9600 bps

8.

9. 13h. Transfer Rate versus PC Bus Speed10.The lowliest XT PC can out-perform the fastest modem transfer rate. For

example: an XT has an 8 bit parallel expansion bus operating at 4.77 MHz. This equates to a data transfer rate of:

11.8 bits x 4.77 MHz = 38.16 Mbps12.Compare this to the fastest modem transfer rates of 57.6 kbps!

13. 14. Physical Connection14.The physical connection determines how many bits (1's or 0's) can be

transmitted at a single instance of time. If only 1 bit of information can be transmitted over the data transmission medium at a time then it is considered a Serial Communication.

15.

16.If more than 1 bit of information is transmitted over the data transmission medium at a time then it is considered a Parallel Communication.

17.

Communications Advantages Disadvantages

Parallel Fast Transfer Rates Short distances only

Serial Long Distances Slow transfer rates

15. Transmission Media - Guided

There are 2 basic categories of Transmission Media: 

Guided and Unguided.

Guided Transmission Media uses a "cabling" system that guides the data signals along a specific path. The data signals are bound by the "cabling" system. Guided Media is also known as Bound Media. Cabling is meant in a generic sense in the previous sentences and is not meant to be interpreted as copper wire cabling only.

Unguided Transmission Media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and as such are often called Unbound Media.

There 4 basic types of Guided Media:

Open Wire Twisted Pair Coaxial Cable Optical Fibre  

15a. Open Wire

Open Wire is traditionally used to describe the electrical wire strung along power poles. There is a single wire strung between poles. No shielding or protection from noise interference is used. We are going to extend the traditional definition of Open Wire to include any data signal path without shielding or protection from noise interference. This can include multiconductor cables or single wires. This media is susceptible to a large degree of noise and interference and consequently not acceptable for data transmission except for short distances under 20 ft.  

 

 15b. Twisted Pair

The wires in Twisted Pair cabling are twisted together in pairs. Each pair would consist of a wire used for the +ve data signal and a wire used for the -ve data signal. Any noise that appears on 1 wire of the pair would occur on the other wire. Because the wires are opposite polarities, they are 180 degrees out of phase (180 degrees - phasor definition of opposite polarity). When the noise appears on both wires, it cancels or nulls itself out at the receiving end. Twisted Pair cables are most effectively used in systems that use a balanced line method of transmission: polar line coding (Manchester Encoding) as opposed to unipolar line coding (TTL logic). 

The degree of reduction in noise interference is determined specifically by the number of turns per foot. Increasing the number of turns per foot reduces the noise interference. To further improve noise rejection, a foil or wire braid shield is woven around the twisted pairs. This "shield" can be woven around individual pairs or around a multi-pair conductor (several pairs).

Cables with a shield are called Shielded Twisted Pair and commonly abbreviated STP. Cables without a shield are called Unshielded Twisted Pair or UTP. Twisting the wires together results in a characteristic impedance for the cable. A typical impedance for UTP is 100 ohm for Ethernet 10BaseT cable.

UTP or Unshielded Twisted Pair cable is used on Ethernet 10BaseT and can also be used with Token Ring. It uses the RJ line of connectors (RJ45, RJ11, etc..)

STP or Shielded Twisted Pair is used with the traditional Token Ring cabling or ICS - IBM Cabling System. It requires a custom connector. IBM STP (Shielded Twisted Pair) has a characteristic impedance of 150 ohms.

15c. Coaxial Cable

Coaxial Cable consists of 2 conductors. The inner conductor is held inside an insulator with the other conductor woven around it providing a shield. An insulating protective coating called a jacket covers the outer conductor.

The outer shield protects the inner conductor from outside electrical signals. The distance between the outer conductor (shield) and inner conductor plus the type of material used for insulating the inner conductor determine the cable properties or impedance. Typical impedances for coaxial cables are 75 ohms for Cable TV, 50 ohms for Ethernet Thinnet and Thicknet. The excellent control of the impedance characteristics of the cable allow higher data rates to be transferred than Twisted Pair cable.

15d. Optical Fibre

Optical Fibre consists of thin glass fibres that can carry information at frequencies in the visible light spectrum and beyond. The typical optical fibre consists of a very narrow strand of glass called the Core. Around the Core is a concentric layer of glass called the Cladding. A typical Core diameter is 62.5 microns (1 micron = 10-6 meters). Typically Cladding has a diameter of 125 microns. Coating the cladding is a protective coating consisting of plastic, it is

called the Jacket.

An important characteristic of Fibre Optics is Refraction. Refraction is the characteristic of a material to either pass or reflect light. When light passes through a medium, it "bends" as it passes from one medium to the other. An example of this is when we look into a pond of water.

(See image 1 below)

If the angle of incidence

is small, the light rays are reflected and do not pass into the water. If the angle of incident is great, light passes through the media but is bent or refracted.

(See image 2 below)

Optical Fibres work on the principle that the core refracts the light and the cladding reflects the light. The core refracts the light and guides the light along its path. The cladding reflects any light back into the core and stops light from escaping through it - it bounds the media!

Optical

Transmission Modes

There are 3 primary types of transmission modes using optical fibre.

They are

a) Step Index b) Grade Index c) Single Mode

 

Step Index has a large core the light rays tend to bounce around, reflecting off the cladding, inside the core. This causes some rays to take a longer or shorted path through the core. Some take the direct path with hardly any reflections while others bounce back and forth taking a longer path. The result is that the light rays arrive at the receiver at different times. The signal becomes longer than the original signal. LED light sources are used. Typical Core: 62.5 microns.

Step Index Mode

Grade Index has a gradual change in the Core's Refractive Index. This causes the light rays to be gradually bent back into the core path. This is represented by a curved reflective path in the attached drawing. The result is a better receive signal than Step Index. LED light sources are used. Typical Core: 62.5 microns.

Grade Index Mode

Note: Both Step Index and Graded Index allow more than one light source to be used (different colours simultaneously!). Multiple channels of data can be run simultaneously!

Single Mode has separate distinct Refractive Indexes for the cladding and core. The light ray passes through the core with relatively few reflections off the cladding. Single Mode is used for a single source of light (one colour) operation. It requires a laser and the core is very small: 9 microns.

Single ModeComparison of Optical Fibres

(See image below)

The Wavelength of the light sources is measured in nanometers or 1 billionth of a meter. We don't use frequency to talk about speed any more, we use wavelengths instead.

Indoor cable specifications:

LED (Light Emitting Diode) Light Source 3.5 dB/Km Attenuation (loses 3.5 dB of signal per kilometre)

850 nM - wavelength of light source

Typically 62.5/125 (core dia/cladding dia)

Multimode - can run many light sources.

Outdoor Cable specifications:

Laser Light Source 1 dB/Km Attenuation (loses 1 dB of signal per kilometre)

1170 nM - wavelength of light source

Monomode (Single Mode)

Advantages of Optical Fibre:

Noise immunity: RFI and EMI immune (RFI - Radio Frequency Interference, EMI -ElectroMagnetic Interference)

Security: cannot tap into cable.

Large Capacity due to BW (bandwidth)

No corrosion

Longer distances than copper wire

Smaller and lighter than copper wire

Faster transmission rate

Disadvantages of Optical Fibre:

Physical vibration will show up as signal noise!

Limited physical arc of cable. Bend it too much & it will break!

Difficult to splice

The cost of optical fibre is a trade-off between capacity and cost. At higher transmission capacity, it is cheaper than copper. At lower transmission capacity, it is more expensive.

15e. Media versus BandwidthThe following table compares the usable bandwidth between the different Guided Transmission

Media

Cable Type Bandwidth

Open Cable 0 - 5 MHz

Twisted Pair 0 - 100 MHz

Coaxial Cable 0 - 600 MHz

Optical Fibre 0 - 1 GHz

16. Transmission Media - UnguidedUnguided Transmission Media is data signals that flow through the air. They are not guided or bound to a channel to follow. They are classified by the type of wave propagation.

16a. RF Propagation

There are 3 types of RF (Radio Frequency) Propagation:

Ground Wave, Ionospheric and

Line of Sight (LOS) Propagation.

Ground Wave Propagation follows the curvature of the Earth. Ground Waves have carrier frequencies up to 2 MHz. AM radio is an example of Ground Wave Propagation.

  

Ionospheric Propagation bounces off of the Earths Ionospheric Layer in the upper atmosphere. It is sometimes called Double Hop Propagation. It operates in the frequency range of 30 - 85 MHz. Because it depends on the Earth's ionosphere, it changes with weather and time of day. The signal bounces off of the ionosphere and back to earth. Ham radios operate in this range. (See image 1 below)

Line of Sight Propagation transmits exactly in the line of sight. The receive station must

be in the view of the transmit station. It is sometimes called Space Waves or Tropospheric Propagation. It is limited by the curvature of the Earth for ground based stations (100 km: horizon to horizon). Reflected waves can cause problems. Examples of Line of Sight Propagation are: FM Radio, Microwave and Satellite.

 

 

 

 

 

16b. Radio Frequencies(see table below)

Radio Frequencies are in the range of 300 kHz to 10 GHz. We are seeing an emerging technology called wireless LANs. Some use radio frequencies to connect the workstations together, some use infrared technology.

 

16c. Microwave

Microwave transmission is line of sight transmission. The Transmit station must be in visible contact with the receive station. This sets a limit on the distance between stations depending on the local geography. Typically the line of sight due to the Earth's curvature is only 50 km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across

the country.

(see image below)

Radio frequencies

The frequency spectrum operates from 0 Hz (DC) to Gamma Rays (1019 Hz).

Name Frequency (Hertz) Examples

Gamma Rays 10^19 +

X-Rays 10^17

Ultra-Violet Light 7.5 x 10^15

Visible Light 4.3 x 10^14

Infrared Light 3 x 10^11

EHF - Extremely High Frequencies 30 GHz (Giga = 10^9) Radar

SHF - Super High Frequencies 3 GHz Satellite & Microwaves

UHF - Ultra High Frequencies 300 MHz (Mega = 10^6) UHF TV (Ch. 14-83)

VHF - Very High Frequencies 30 MHz FM & TV (Ch2 - 13)

HF - High Frequencies 3 MHz2 Short Wave Radio

MF - Medium Frequencies 300 kHz (kilo = 10^3) AM Radio

LF - Low Frequencies 30 kHz Navigation

VLF - Very Low Frequencies 3 kHz Submarine Communications

VF - Voice Frequencies 300 Hz Audio

ELF - Extremely Low Frequencies 30 Hz Power Transmission

 

Microwaves operate at high operating frequencies of 3 to 10 GHz. This allows them to carry large quantities of data due to the large bandwidth.

Advantages:

a. They require no right of way acquisition between towers.b. They can carry high quantities of information due to their high operating

frequencies.

c. Low cost land purchase: each tower occupies small area.

d. High frequency/short wavelength signals require small antenna.

Disadvantages:

a. Attenuation by solid objects: birds, rain, snow and fog.b. Reflected from flat surfaces like water and metal.

c. Diffracted (split) around solid objects

d. Refracted by atmosphere, thus causing beam to be projected away from receiver.

16d. SatelliteSatellites are transponders that are set in a geostationary orbit directly over the equator. A transponder is a unit that receives on one frequency and retransmits on another. The geostationary orbit is 36,000 km from the Earth's surface. At this point, the gravitational pull of the Earth and the centrifugal force of Earths rotation are balanced and cancel each other out. Centrifugal force is the rotational force placed on the satellite that wants to fling it out to space.

The uplink is the transmitter of data to the satellite. The downlink is the receiver of data. Uplinks and downlinks are also called Earth stations due to be located on the Earth. The footprint is the "shadow" that the satellite can transmit to. The shadow being the area that can receive the satellite's transmitted signal.

16e. Iridium Telecom System

The Iridium telecom system is a new satellite sytem that will be the largest private aerospace project. It is a mobile telecom system to compete with cellular phones. It relies on satellites in Lower Earth Orbit (LEO). The satellites will orbit at an altitude of 900 - 10,000 km and are a polar non-stationary orbit. They are planning on using 66 satellites. The user's handset will require less power and will be cheaper than cellular phones. There will be 100% coverage of the Earth.

They were planning to launch starting 1996-1998 and having 1.5 million subscribers by end of the decade. Unfortunately at the time of this writing, the Iridium project looked very financially unstable.

17. RS-232D Serial Interface StandardThe RS-232D Serial Interface Standard added the mechanical characteristics to the RS-232C Standard. The RS-232D standard defines:

The Mechanical Characteristics of the Interface The Electrical Characteristics of the Interface

The Function of Each Signal

Subsets of the Signals for Certain Applications

The European version of RS-232D is defined in:

V.24 - Mechanical Standard

V.28 - Electrical Standard

17a. Mechanical Characteristics of the RS-232D

Mechanical Characteristics of the RS-232D Interface defines:i. The connector is a DB25 connector. DB9 is not universally accepted.

ii. The connector gender is Male at the DTE and Female at the DCE.

iii. The assignments of signals to pins

iv. The maximum cable length is 50 ft.

v. The maximum cable capacitance = 2500 pF. Typical cable has 50 pF/foot capacitance.

17b. Electrical Characteristics of the RS-232D

Electrical Characteristics of the RS-232D Interface defines:

The transmitter side generates a voltage between +5 and +25 Volts for a Space (digital 0 or Low) and generates a voltage between -5 and -25 Volts for a Mark (digital 1 or High).

Introduction to Data Communications17b. Electrical Characteristics of the RS-232D (cont'd)

The receiving side recognizes a Space (digital 0 or Low) as any voltage between +3 and +25V and a Mark (digital 1 or

High) as any voltage between -3 and -25V. The standard allows for a voltage loss through the cable and noise immunity by reducing the receive requirements to +/-3 Volts!

 

17c. Function of Each Signal

Pin Name Description EIA Circuit1 GND Chassis ground AA2 TXD Transmit Data (TXD) BA3 RXD Receive Data (RXD) BB4 RTS Ready to Send CA5 CTS Clear to Send CB6 DSR Data Set Ready (DCE Ready) CC7 SGND Signal ground AB

8 DCD Carrier Detect (CD or RLSD)(RLSD - Received Line Signal Detector) CF

9 n/u10 n/u11 n/u12 DCD2 Secondary Carrier Detect (SRLSD) SCF13 CTS2 Secondary Clear to Send SCB14 TXD2 Secondary Transmit Data SBA15 TxSigC Transmitter Signal Element Timing - DCE DB16 RXD2 Secondary Receive Data SBB17 RxSig Receive Signal Element Timing - DCE DD

18 LL Local Loopback19 RTS2 Secondary Ready to Send SCA20 DTR Data Terminal Ready (DTE Ready) CD21 SQ/RL Signal Quality/Remote Loopback CG22 RI Ring Indicator CE23 DSRS Data Signal Rate Selector CH/CI24 TxSigT Transmitter Signal Element Timing - DTE DA25 TM Test Mode

The signals in Bold/Italic are required for a basic asynchronous modem connection.

17d. Subsets of Signals for Certain Applications

Data Signals

2 TXD Transmit Data Data generated by DTE BA3 RXD Receive Data Data generated by DCE BB

Control Signals

4 RTS Ready to Send DTE wishes to transmit5 CTS Clear to Send DCE ready to receive6 DSR Data Set Ready (DCE Ready) DCE powered on & ready to go20 DTR Data Terminal Ready (DTE Ready) DTE powereded on & ready to go22 RI Ring Indicator Phones ringing

Test Modes

18 LL Local Loopback Initiate Local Loopback Self-Test21 SQ/RL Signal Quality/Remote Loopback Initiate Remote Loopback Self-Test25 TM Test Mode Initiate Test Mode

Synchronous Control Signals

21 SQ/RL Signal Quality/Remote Loopback Error in received data!23 DSRS Data Signal Rate Selector DTE can dynamically select 1 of 2 data rates

Synchronous Timing Signals

15 TxSigC Transmitter Signal Element Timing - DCE DCE generated17 RxSig Receive Signal Element Timing - DCE DCE generated24 TxSigT Transmitter Signal Element Timing - DTE DTE generated

Ground/Shield

1 GND Chassis ground Shield DTE side only for noise protection.Do NOT connect to signal ground!

7 SGND Signal ground Signal return path

18. RS-232D Flow ControlFlow control is the communication between the data transmitter and data receiver to determine whose turn it is to talk. Another name for flow control is handshaking. Flow control is the exchange of predetermined codes and signals between two devices to establish and maintain a connection.

Modem flow control is used between the PC and modem to determine if the modem is ready to receive data from the terminal, if carrier is present, if the line is ringing, etc....

Source to Destination (End to End) flow control

Source to destination flow control is used to control the data communication from the sendor to the receiver. It may or may not have a modem involved. Source to destination may involve direct PC to PC communication or PC to Serial Printer communication. It is also called end to end flow control.

DTE-DCE Flow Control

There are 2 basic types of DTE-DCE Flow Control used with RS-232D connections:

Hardware handshaking Software handsh

18a. Hardware Handshaking

Hardware Handshaking uses the physical signals in the RS-232D cable such as RTS, CTS, DSR and TSR to control the flow of data. Hardware Handshaking is used primarily with modems: PC to modem connection or terminal to modem connection.PC to Modem Handshaking (DTE-DCE)

The basic signals required for DTE-DCE Hardware Handshaking are:

The following diagram indicates the signals used when two PCs are communicating using hardware handshaking.

The procedure for connecting between 2 PCs using modems and the telephone line is as follows:

DTE(Tx) is originating the call and DTE(Rx) is answering the call. DTE(Rx) is in the auto-answer mode with DTR(Rx) and DSR(Rx) High is ready to answer a call.

1. The communication program controls the handshaking. The DTE (Tx) dials the phone number:

a. PC sends DTR(Tx) - PC is awake!b. Modem replies with DSR(Tx) - Modem is awake, too!c. PC sends RTS(Tx) - Ready for some data?d. Modem replies with CTS(Tx) - Okay send away!e. PC transmits data on TXD - Initialize dial this telephone number.

2. DTE(Rx) is in the auto-answer mode with DTR(Rx) and DSR(Rx) High, indicating the receive end is ready to answer a call. This has been setup by the communication

program similar to dialing the number in the previous step except the modem is told to go to auto-answer mode. The phone rings:a. Modem sends RI(Rx) - Hey the phone's ringing!b. Modem picks up phone linec. Modem sends answer carrier

The modem since it was initialized in the auto-answer mode, picks up the phone line and sends Answer Carrier (2125 Hz). Everytime the phone rings, the RI line goes high. The communication program will usually display the word "ring" on the screen.

3. Back at the Transmit End:a. Modem sends CD(Tx) - We're connected and they are sending us good carrier!b. PC sends RTS(Tx) - Okay send them our carrier (1170 Hz).c. Modem waits - Delay so that Rx modem can lock to the carrierd. Modem sets CTS(Tx) - Okay now we should be ready to send datae. PC sends TxD(Tx) - Here's some data to send over.

4. At the Receive End:

a. Modem sends CD(Rx) - We're connected & they're sending good carrier (1170 Hzb. Modem sends Rxd(Rx) - Here's some data for you.

The communication program then interprets the data and decides if a reply is required or if more data is coming. The communication programs handle the transfer of the data and the direction.

5. Both Originate or Answer can end the communication:

a. DTE(Tx) drops RTS(Tx) or DTR(Tx) - I'm done, hang-up the phone.

b. DCE(Tx) modem drops DSR(Tx) and the Carrier (1170 Hz) - I've disconnected.c. DCE(Rx) modem drops CD(Rx) - No carrier, they're hanging upd. DTE(Rx) drops RTS(Rx) - Hang up on theme. DCE(Rx) modem drops DSR(Rx) and the Carrier (2125 Hz) - I've disconnected.

18b. Hardware Null Modems

Null modems are a way of connecting 2 DTEs together without using a modem - we are nulling out the modems. This gives way to the term Null Modem. When 2 DTEs are connected together, the TXD Pin2 of one DTE is crossed to Pin 3 RXD of the other DTE. We also have to fool the DTEs into believing that they are connected to DCE devices. This is done by crossing the control lines as follows:

Notice that RI (Ring Indicator) and CD (Carrier Detect) are not used when connecting directly from DTE to DTE. They are a function of a telephone system and by nulling out the modems, we've eliminated the telephone system. This can cause problems when transferring files directly because most communication programs detect loss of

carrier (CD) as a disconnect command. The communication program will abort the data transfer if CD is not present. This can usually be over-ridden by de-selecting "Transfer Aborted if CD Lost" (or something similar) in one of the communication software configuration menus.

18c. Software Handshaking (Xon/Xoff)

Software Handshaking does not use the RS-232D control signals, it uses the software commands Xon/Xoff to control the data flow. Do not use software handshaking with a modem, because you will lose several important function of the modem such as: RI, and CD.Xon Transmit On - ASCII Character DC1Xoff Transmit Off - ASCII Character DC2

Software handshaking is a simple flow control method that is used mainly with DTE to DTE and DTE to Serial Printer connections. The receiving device controls the flow of data by issuing Xon (okay to transmit data) commands and Xoff (stop - let me catch up) commands. A good example is the DTE to Serial Printer connection.

For example, a dot-matrix printer cannot physically print faster than a transfer rate of 300 bps. Printers are usually equipped with a memory buffer to store the data before it is printed. The printer buffer allows large chunks of data to be downloaded to the printer from the DTE, thereby freeing up the DTE to do other tasks rather than wait for a page to be printed.

When the data is first being downloaded to the printer, the printer issues a Xon command to the DTE. As the print buffer becomes full (90%), the printer issues an Xoff command to stop transmitting data until the printer catches up. When the print buffer becomes almost empty (20%) than the printer issues a Xon command. This goes on until the complete document is printed.

18d. Software Null Modem

Since we are using software to control the data flow, we can eliminate a few of the control lines used in the Hardware Null Modem cable. In its simplest form, the Null Modem cable consists of SGND, and the TXDs & RXDs crossed.

Usually we find that we have to add a few control lines to fool the DTE's hardware. There is no standard Software Null Modem configuration for Xon/Xoff. The exact connection will vary from device manufacturer to device manufacturer.

18e. Terminals & PCs

Terminals are considered dumb devices. They can only display data on the screen and input data from a keyboard. They communicate with a mainframe or minicomputer which does the number crunching and work. Terminals do not have hard-drives for storing files or RAM for running programs. Terminals cannot work by themselves, they are an extension of the mainframe or minicomputer's display and keyboard.

PCs have microprocessors which are the smarts or brains that can do number crunching and work. They have hard-drives for storage and RAM for running programs. They are stand-alone devices.

The purpose of communication programs like Procomm Plus, Kermit, PCLink or Quicklink II is to turn your PC into a terminal. It is the computer world's equivalent of a lobotomy.

19. TimingTiming refers to how the receiving system knows that it received the start of a group of bits and the end of a group of bits. Two major timing schemes are used: Asynchronous and Synchronous Transmission.

i. Asynchronous Transmission sends only 1 character at a time. A character being a letter of the alphabet or number or control character. Preceding each character is a Start bit and ending each character is 1 or more Stop bits.

ii. Synchronous Transmission sends packets of characters at a time. Each packet is preceded by a Start Frame which is used to tell the receiving station that a new packet of characters is arriving and to synchronize the receiving station's internal clock. The packets also have End Frames to indicate the end of the packet. The packet can contain up to 64,000 bits. Both Start and End Frames have a special bit sequence that the receiving station recognizes to indicate the start and end of a packet. The Start and End frames may be only 2 bytes each.

Packet

Conventional representation has asynchronous data flowing left to right and synchronous data flowing right to left.

19a. Asynchronous vs. Synchronous Transmission

Asynchronous transmission is simple and inexpensive to implement. It is used mainly with Serial Ports and dialup connections. Requires start and stop bits for each character - this adds a high overhead to transmission. For example: for every byte of data, add 1 Start Bit and 2 Stop Bits. 11 bits are required to send 8 bits! Asynchronous is used in slow transfer rates typically up to 56 kbps.

Synchronous transmission is more efficient as little as only 4 bytes (3 Start Framing bytes and 1 Stop Framing byte) are required to transmit up to 64 kbits. Synchronous transmission is more difficult and expensive to implement. It is used with all higher comunication transfer rates: Ethernet, Token Ring etc... Synchronous is used in fast transfer rates typically 56 kbps to 100 Mbps.

Historically, synchronous communications were operating over 2400/4800 baud modems on point-to-point communications, for example: IBM2770/IBM2780/IBM3780 (historical information courtesy of Jacques Sincennes, University of Ottawa)

Example: Compare a 10K Byte data transmission using Asynchronous transmission & Synchronous Transmission. Determine the efficiency (10 kBytes = 80 kbits).

Asynchronous: Add 3 bits (1 Start and 2 Stop bits) for every byte transmitted.

80 kbits + 30 kbits = total of 110 kbits transmitted

Synchronous: Add 4 bytes (32 bits) for the complete 10K byte data packet.

80 kbits + 32 bits = total of 80.032 kbits transmitted

efficiency = data transmitted x 100 = 80 kbits x 100 = 99.9%

Transmission Advantages DisadvantagesAsynchronous Simple & Inexpensive High Overhead

Synchronous Efficient Complex and Expensive

20a. Start/Stop bits

The purpose of the Start bit is to notify the receiving station of a new character arriving. Typically data is shown moving left to right. This is how it would appear on a Storage Oscilloscope or Network Analyser. The MSB ( Most Significant Bit) is sent first and the LSB (Least Significant Bit) is sent last.

The purpose of the Stop bits is to indicate the end of data. There could be 1 or 2 stop bits with 1 being the typical number of stop bits used today. In Asynchronous transmission, the characters are sent individually with a quiet period in between (quiet meaning 0 bit level). Asynchronous communications requires the transmitting station and the receiving station to have individual internal free-running clocks operating at the same frequency. Free-running means that the clocks are not locked together.

Both clocks operating at same frequency:

The receive station starts checking for data after the Start bit is received (Start bit is a wake up call!).

The receive station samples the transmitted data in the middle of each data bit. The samples are evenly spaced and match the transmitted data because both transmit and receive clocks are operating at the same frequency.

Receive clock frequency higher than transmitted frequency:

If the receive station's clock is higher in frequency, the samples will be spaced closer together (higher frequency - shorter period). In the above example, we transmitted the following data: 0100 1010 but we received the data: 0100 0101. The samples are out of synchronization with the transmitting data. We would have an error in receiving data.

Clocks are controlled by crystals (abbreviated: Xtal). Crystals are metal cans that hold a piezo-electric element that resonates at a certain frequency when a voltage is applied to it. If you drop a crystal or a printed circuit board (PCB) that has a crystal on it, the crystal can fracture inside the metal can. Either it will stop working or change its frequency, both result in a malfunctioning circuit! Crystals are also temperature sensitive and change frequency with temperature!

Receive clock frequency lower than transmitted frequency:

If the receiving station's clock is lower in frequency than the transmitted frequency, then the samples become farther apart (lower frequency - wider period). Again the samples become out of sync with the transmitted data!

The transmitted data is 0100 1010 but the receive data is 0101 0101! Again we would have receive data errors.

This is a basic problem with asynchronous communications, both transmitter and receiver require a very stable clock to work properly. At high frequencies (which result in high transfer rates), clock stability is critical and asynchronous transmission

is very difficult to accomplish. Because of this inherent problem with asynchronous transmission, it is used at low frequency/slow transfer rates.

0b. 7/8 Bit Codes

There are 2 common data transfer codes in data communication:a. 7 bit code (Text)b. 8 bit (Binary)

7 Bit Code or Text:

7 bit data code transfer is used to transfer text files. These are files consisting of ASCII text characters only. There are only 27 or 128 different characters in the ASCII text transfer type.

Usually, files that are meant to be read by the human eye used 7 bit code! Text editors like DOS's EDLIN & EDITOR or Unix's pico or vi are used to change or modify the files. Examples of text files: autoexec.bat, config.sys, .signature, E-mail, stories, information.

8 Bit Code or Binary:

8 bit code is used to transfer binary files that contain information that is to be "read" specifically by an application or microprocessor. They contain 8 bit (1 byte) control codes and have 28 or 256 different characters. Examples of binary files are: drawings.bmp (bit mapped graphics), win.com (application), newtext.zip (compressed files).

Common Problems:

If you download a binary (8 bit) file, using text (7 bit) mode, you lose 1 bit from each character. In a binary file this is disastrous! The text transfer mode ignores the 8th bit and discards it into the bit bucket. In the following example the number 202 is transmitted but the number 74 is received. You end up with a corrupted file!

Decimal BinaryTransmitted 202 1100 1010 - 8 bit dataReceived 74 100 1010 - 7 bit data (MSB is ignored)

If you download a text file (7 bit) using binary (8 bit) mode, an extra bit is inserted into the data. The bit is set to 0 and placed as the MSB or 8th bit.

Decimal Binary

Transmitted 74 100 1010 - 7 bit dataReceived 74 0100 1010 - 8 bit data

The received file works beautifully! If there is a choice or you are not sure what the number of data bits are, always pick Binary or 8 bit transfer mode! Originally, when transfer rates were very slow (300 to 1200 bps), sending 7 or 8 bits would make a big difference in transfer times.

20c. Parity Bits

In asynchronous communications, a simple error checking method is used: Parity Checking. There are 3 types of Parity Bits: Even, Odd and None. None means that there is no Parity Checking and the Parity Checking is disabled!

Even Parity Generation

Even Parity counts the number of 1s in the data to see if the total is an even number. If the number of 1s is an even number then the Parity bit is set to 0. If the number of 1s is an odd number, then the Parity bit is set to 1 to make the total number of 1s an even number. The Even Parity Bit is used to make the total number of 1s equal to an even number.

Data Even Parity Bit0100 1010 1 3 x 1s in Data: 3 is an odd number, Parity Bit = 10111 1110 0 6x 1s in Data: 6 is an even number, Parity Bit = 01010 1010 ? What should the parity bit be?

Even Parity Checking

When a data with even parity is received. The number of 1s in both the data and the parity bit are counted. If the number of 1s is an even number than the data is good data, if it is an odd number than the data is corrupted.

Data Even Parity Bit0100 1010 1 4 x 1s in data and parity bit = Good data0111 1110 1 7 x 1s in data and parity bit = Bad data1010 1010 0 Is this good or bad data?

Odd Parity Generation

Odd Parity is the opposite of Even Parity. Odd Parity counts the number of 1s in the data to see if the total is an odd number. If the number of 1s is an odd number then the Parity bit is set to 0. If the

number of 1s is an even number, then the Parity bit is set to 1 to make the total number of 1s an odd number. The Odd Parity Bit is used to make the total number of 1s equal to an odd number.

Data Odd Parity Bit0100 1010 1 3 x 1s in Data: 3 is an odd number, Parity Bit = 00111 1110 0 6x 1s in Data: 6 is an even number, Parity Bit = 11010 1011 ? What should the parity bit be?Odd Parity Checking

When a data with odd parity is received. The number of 1s in both the data and the parity bit are counted. If the number of 1s is an odd number than the data is good data, if it is an even number than the data is corrupted.

Data Odd Parity Bit0100 1010 0 3 x 1s in data and parity bit = Good data0111 1110 0 6 x 1s in data and parity bit = Bad data1010 1010 0 Is this good or bad data?

Parity Agreement

Both receive and transmit stations must agree on the type of parity checking used before transmitting. Usually it is setup in the communications parameters setup. Most common transfer are: 8n1 (8 data bits, no parity, 1 stop bit) or 7e2 (7 data bits, even parity, 2 stop bits).

The parity bit is added in the asynchronous bit stream just before the stop bits and adds to the overhead for asynchronous transmission. A total of 12 bits must be transmitted in order to send 8 bits of data.

Problems with Parity Checking

There is a problem with parity checking. It only works reliably if there is only 1 bit error in the transmitted character stream. If there are 2 bit errors, the parity checking may not detect that there is an error. For example:

Data Odd Parity BitTransmitted 0100 1010 0 3 x 1s in data and parity bit = Good dataReceived 0110 1110 0 5 x 1s in data and parity bit = Good data?

Parity checking would pass the received data as good data even though 2 bits are corrupted!

21. Line Encoding

The waveform pattern of voltage or current used to represent the 1s and 0s of a digital signal on a transmission link is called line encoding. The common types of line encoding are Polar, Unipolar, Bipolar and Manchester encoding.

21a. Unipolar Encoding

Unipolar encoding has 2 voltage states with one of the states being 0 volts. Since Unipolar line encoding has one of its states being 0 Volts, it is also called Return to Zero (RTZ). A common example of Unipolar line encoding is the TTL logic levels used in computers and digital logic.

Unipolar line encoding works well for inside machines where the signal path is short but is unsuitable for long distances due to the presence of stray capacitance in the transmission medium. On long transmission paths, the constant level shift from 0 volts to 5 volts causes the stray capacitance to charge up (remember the capacitor charging formula 1-e-t/RC !). There will be a "stray" capacitor effect between any two conductors that are in close proximity to each other. Parallel running cables or wires are very suspectible to stray capacitance.

If there is sufficient capacitance on the line and a sufficient stream of 1s, a DC voltage component will be added to the data stream. Instead of returning to 0 volts, it would only return to 2 or 3 volts! The receiving station may not recognize a digital low at voltage of 2 volts!

Unipolar line encoding can have synchronization problems between the transmitter and receiver's clock oscillator. The receiver's clock oscillator locks on to the transmitted signal's level shifts (logic changes from 0 to 1). If there is a long series of logical 1s or 0s in a row. There is no level shift for the receive oscillator to lock to. The receive oscillator's frequency may drift and become unsynchronized. It could lose track of where the receiver is supposed to sample the transmitted data!

Receive oscillator may drift during the period of all 1s

21b. Polar Encoding

When the digital encoding is symmetrical around 0 Volts, it is called a Polar Code. The RS-232D interface uses Polar line encoding. The signal does not return to zero, it is either a +ve voltage or a -ve voltage. Polar line encoding is also called None Return To Zero (NRZ). Polar line encoding is the simplest pattern that eliminates most of the residual DC problem.

There is still a small residual DC problem but Polar line encoding is a great improvement over Unipolar line encoding. Polar encoding has an added benefit in that it reduces the power required to transmit the signal by one-half compared with unipolar.

RS-232D TXD

Polar line encoding has the same synchronization problem as Unipolar line encoding. If there is a long string of logical 1s or 0s, the receive oscillator may drift and become unsynchronized.

21c. Bipolar Line Encoding

Bipolar line encoding has 3 voltage levels, a low or 0 is represented by a 0 Volt level and a 1 is represented by alternating polarity pulses. By alternating the polarity of the pulses for 1s, the residual DC component cancels.

Bipolar Line Encoding

Synchronization of receive and transmit clocks is greatly improved except if there is a long string of 0s transmitted. Bipolar line encoding is also called Alternate Mark Inversion (AMI).

21d. Manchester Line Encoding

In the Manchester Line Encoding, there is a transition at the middle of each bit period. The mid-bit transition serves as a clocking mechanism and also as data: a low to high transition represents a 1 and a high to low transition represents a 0.

Manchester line encoding has no DC component and there is always a transition available for synchronizing receive and transmit clocks. Manchester line encoding is also called a self clocking line encoding. It has the added benefit of requiring the least amount of bandwidth compared to the other line encoding. Manchester line encoding requires 2 frequencies: the base carrier and 2 x the carrier frequency. All others require a range from 0 hertz to the maximum transfer rate frequency.

Manchester line encoding can detect errors during transmission. a transition is expected for during every bit period. The absence of a transition would indicate an error condition.

22. Standard Digital CodesComputers process information in digital form. Characters are assigned a 7 or 8 bit code to indicate which character it is. This 7 or 8 bit code becomes a number (usually hexadecimal) that the computer can work with. The characters stored in a computer include:

Lower case letters: a - zUpper case letters: A - ZDigits: 0 - 9Punctuation Marks: . , ; : ! ? etc...Unit Symbols: # $ % & * etc...Control Codes: EOF, etc..

There are 2 major codes existing today: ASCII (pronounced ah-skee) and EBCDIC (pronounced eb-ce-dic).

22a. EBCDIC - Extended Binary Coded Decimal Interchange Code

EBCDIC is used mainly by IBM mainframes and compatibles. It is not common in the PC LAN world unless you are connecting to the IBM mainframe world. In order to connect, you would require either an IBM 3270 terminal emulation program or a device called a gateway.

Table 18-1 shows the EBCDIC translation table. Computers speak in binary code which is 1s and 0s. The computers do not know what the letter "A" is. Instead they speak of the letter "A" as the binary number 1100 0001. It is not easy for humans to remember binary numbers such as 1100 0001 but it is easier to remember the hexadecimal number C1. The hexadecimal number C1 is equal to the binary number 1100 0001.

The hexadecimal number C1 is equal to the decimal number 193. The table 18-1 shows both the decimal (dec) number and the hexadecimal (hex) number for the capital letter "A". Lower case "a" is represented by the EBCDIC decimal code 129 or hexadecimal code 81.

Besides character codes such as the previous letter "A", the EBCDIC code also defines control characters. These are characters that have special meaning. For example, the control character FF stands for Form Feed and is used by printers to advance one page or to eject a page. The decimal code for FF is 12 and the hexadecimal code is C.

Both hexadecimal and decimal codes are indicated because many times, a program or interface will report the EBCDIC code in one or the other formats. You may have to use Table 18-1 to translate from the numerical code to the actual character.

Note: Some EBCDIC codes are not defined and have no name.

Dec Hex Name Dec Hex Name Dec Hex Name Dec Hex Name

0 0 NUL 32 20 DS 64 40 RSP 96 60 -

1 1 SOH 33 21 SOS 65 41 97 61 /

2 2 STX 34 22 FS 66 42 98 62

3 3 ETX 35 23 WUS 67 43 99 63

4 4 SEL 36 24 BYP 68 44 100 64

5 5 HT 37 25 LF 69 45 101 65

6 6 RNL 38 26 ETB 70 46 102 66

7 7 DEL 39 27 ESC 71 47 103 67

8 8 GE 40 28 SA 72 48 104 68

9 9 SPS 41 29 SFE 73 49 105 6910 A RPT 42 2A SM 74 4A ¢ 106 6A |11 B VT 43 2B CSP 75 4B . 107 6B ,12 C FF 44 2C MFA 76 4C < 108 6C %13 D CR 45 2D ENQ 77 4D ( 109 6D -14 E SO 46 2E ACK 78 4E + 110 6E >15 F SI 47 2F BEL 79 4F ê 111 6F ?16 10 DLE 48 30 80 50 & 112 7017 11 DC1 49 31 81 51 113 7118 12 DC2 50 32 SYN 82 52 114 7219 13 DC3 51 33 IR 83 53 115 7320 14 RES 52 34 PP 84 54 116 7421 15 NL 53 35 TRN 85 55 117 7522 16 BS 54 36 NBS 86 56 118 7623 17 POC 55 37 EOT 87 57 119 7724 18 CAN 56 38 SBS 88 58 120 7825 19 EM 57 39 IT 89 59 121 79 `26 1A UBS 58 3A RFF 90 5A ! 122 7A :27 1B CU1 59 3B CU3 91 5B $ 123 7B #28 1C IFS 60 3C NAK 92 5C * 124 7C @29 1D IGS 61 3D 93 5D ) 125 7D '30 1E IRS 62 3E SUB 94 5E ; 126 7E =31 1F IUS 63 3F SP 95 5F ù 127 7F "

Table 18-1 EBCDIC code

Dec Hex Name Dec Hex Name Dec Hex Name Dec Hex Name128 80 160 A0 192 C0 { 224 E0 \129 81 a 161 A1 ~ 193 C1 A 225 E1 NSP130 82 b 162 A2 s 194 C2 B 226 E2 S131 83 c 163 A3 t 195 C3 C 227 E3 T132 84 d 164 A4 u 196 C4 D 228 E4 U133 85 e 165 A5 v 197 C5 E 229 E5 V134 86 f 166 A6 w 198 C6 F 230 E6 W135 87 g 167 A7 x 199 C7 G 231 E7 X136 88 h 168 A8 y 200 C8 H 232 E8 Y137 89 i 169 A9 z 201 C9 I 233 E9 Z138 8A 170 AA 202 CA SHY 234 EA139 8B 171 AB 203 CB 235 EB140 8C 172 AC 204 CC 236 EC141 8D 173 AD 205 CD 237 ED142 8E 174 AE 206 CE 238 EE143 8F 175 AF 207 CF 239 EF144 90 176 B0 208 D0 } 240 F0 0145 91 j 177 B1 209 D1 J 241 F1 1146 92 k 178 B2 210 D2 K 242 F2 2147 93 l 179 B3 211 D3 L 243 F3 3148 94 m 180 B4 212 D4 M 244 F4 4149 95 n 181 B5 213 D5 N 245 F5 5150 96 o 182 B6 214 D6 O 246 F6 6151 97 p 183 B7 215 D7 P 247 F7 7152 98 q 184 B8 216 D8 Q 248 F8 8153 99 r 185 B9 217 D9 R 249 F9 9154 9A 186 BA 218 DA 250 FA155 9B 187 BB 219 DB 251 FB156 9C 188 BC 220 DC 252 FC157 9D 189 BD 221 DD 253 FD158 9E 190 BE 222 DE 254 FE159 9F 191 BF 223 DF 255 FF EO

Table 18-1 EBCDIC code (cont'd)

22b. ASCII - American Standard Code for Information Interchange

ASCII is the most popular code and is used by the majority of the computing world. ASCII itself is a 7 bit code which allows only 128 characters (27). Most applications follow IBM's Extended ASCII code which uses 8 bits and allows an addition 128 graphic characters for a total of 256 characters (28). We will be concentrating on 7 bit ASCII codes.

Format effectors

Format effectors control the movement of the cursor on the screen and the print head in a printer. The format effectors are:

BS BackspaceHT Horizontal TabLF Line FeedCR Carriage ReturnFF Form FeedVT Vertical Tab

Communication Controls

Communication Controls are used in controlling data transmission over a communication network. They are used in both Asynchronous and Synchronous Transmissions. They are used in "handshaking".

STX Start of TextETX End of TextEOT End of TransmissionENQ End of InquiryACK AcknowledgeNAK Negative AcknowledgeEXT InterruptSYN Synchronous idleETB End of BlockEOF End of File

Information Separators

Information separators are used to separate database enquiries and files:

FS File Separator (in a PC - used as cursor R, L, U, D)

GS Group SeparatorRS Record SeparatorUS Unit Separator

Additional Control Codes

Of the remaining codes used by the computer, the most important ones are:

NUL Nothing characterBEL Rings the bell!DC1 - 4 Device Control 1 - 4ESC Escape - used for formatting printers & terminalsDEL Delete - deletes characters under cursor

DC1 & DC2 are used in the Xon/Xoff software handshaking to control data transfer.

Displaying ASCII codes directly to the screen

You can type in the ASCII codes directly to the screen on IBM capatible computers. You press the "ALT" key and a 3 digit number on the numeric keypad. The 3 digit number is the ASCII decimal code for the character. You must use the numeric keypad, the QWERTY numbers will NOT work.

For example, the character "A" corresponds to the ASCII decimal code 65. To access the ASCII code directly, hold down the ALT key and type in 065 on the numeric keypad. On releasing the ALT key, the letter A will appear on the screen.

Table 18-2 shows the ASCII codes according to decimal numbers and hexadecimal numbers. If a network sniffer or analyzer is used, it will show raw data in decimal or hexadecimal formats. You may have to perform a manual translation using Table 18-2.

Dec Hex Name Dec Hex Name Dec Hex Name Dec Hex Name0 0 NUL 32 20 Space 64 40 @ 96 60 `1 1 SOH 33 21 ! 65 41 A 97 61 a2 2 STX 34 22 " 66 42 B 98 62 b3 3 ETX 35 23 # 67 43 C 99 63 c4 4 EOT 36 24 $ 68 44 D 100 64 d5 5 ENQ 37 25 % 69 45 E 101 65 e6 6 ACK 38 26 & 70 46 F 102 66 f7 7 BEL 39 27 ¢ 71 47 G 103 67 g8 8 BS 40 28 ( 72 48 H 104 68 h

9 9 HT 41 29 ) 73 49 I 105 69 i10 A LF 42 2A * 74 4A J 106 6A j11 B VT 43 2B + 75 4B K 107 6B k12 C FF 44 2C , 76 4C L 108 6C l13 D CR 45 2D - 77 4D M 109 6D m14 E S0 46 2E . 78 4E N 110 6E n15 F S1 47 2F / 79 4F O 111 6F o16 10 DLE 48 30 0 80 50 P 112 70 p1 11 DC1 49 31 1 81 51 Q 113 71 q18 12 DC2 50 32 2 82 52 R 114 72 r19 13 DC3 51 33 3 83 53 S 115 73 s20 14 DC4 52 34 4 84 54 T 116 74 t21 15 NAK 53 35 5 85 55 U 117 75 u22 16 SYN 54 36 6 86 56 V 118 76 v23 17 ETB 55 37 7 87 57 W 119 77 w24 18 CAN 56 38 8 88 58 X 120 78 x25 19 EM 57 39 9 89 59 Y 121 79 y26 1A SUB 58 3A : 90 5A Z 122 7A z27 1B ESC 59 3B ; 91 5B [ 123 7B {28 1C FS 60 3C < 92 5C \ 124 7C |29 1D GS 61 3D = 93 5D ] 125 7D }30 1E RS 62 3E > 94 5E ^ 126 7E ~31 1F US 63 3F ? 95 5F _ 127 7F DEL

Table 18-2 ASCII code

23. Voice Channel CommunicationsThe voice channel or dial-up line is the line from our telephone/modem to the outside world.

As the name implies "voice" channel is designed to carry human speech over the telephone wires.

23a. Voice Channel Specification

Human speech covers the frequency range of 100 to 7000 Hz (hertz) but research has shown that the intelligence part of human speech is carried in the 300 - 3400 Hz range. This range is called the Voice Band.

The Voice Channel has a range of 0 to 4 kHz (4000 Hz). The area from 3400 to 4000 Hz is used for system control and is called Out of Band Signalling.

23b. Voice Channel Constraints

Due to the limited Bandwidth (BW) of the Voice Channel (0-4 kHz), we are limited to the amount of data that we can pass through the Voice Channel. The Nyquist Theorem addresses this limitation.

23c. Nyquist Theorem

In a digital Public phone system, the signal leaving our telephone at our house is an analog signal. It goes to the Central Office through the Local Loop. The Local Loop is the name for the wires that run from our house to the Central Office. The Central Office (also called a local exchange) is the building that all the neighbourhood phones with the same local connect. A local is the 1st 3 digits of your 7 digit phone number or LDN (Listed Directory Number).

At the Central Office, the analog signal is converted into a digital signal consisting of 1s and 0s.

The Nyquist Theorem states that to accurately reproduce an analog signal with a digital signal, the analog signal must be sampled a minimum of 2x the highest frequency of the analog signal.

This means that for the Voice Channel (0 to 4 kHz) to be digitized, we must sample the Voice Channel at 2x the highest frequency (4 kHz) which would be 8 kHz. This means that as soon as you digitize an analog signal, you must immediately double the bandwidth.

24. Telephone NetworksThe telephone network consists of your phone at home, that is connected by the Local Loop to the Central Office which is connected to a Hierarchical Phone Network. Worldwide there are over 300 million (300,000,000) telephones - 98% of them interconnected.

24a. POTS - Plain Old Telephone Set

The POTS or Plain Old Telephone Set consists of 5 sections:i. Ringer Unit

ii. Hook Switch

iii. Dialer Unit

iv. Hybrid/Speech Network

v. Hand Set

The connection to the CO (Central Office) is with only 2 wires: Tip and Ring. This connection is called the Local Loop.

The Tip is +ve and coloured green.. The Ring is -ve and coloured Red. If you look at a phone jack in your house, you will see that it is wired for 4 wires: Red, Green, Black and Yellow. Black and Yellow are not normally used.

The black and yellow wires can be used for a second telephone line or they can be used for running a Network Physical layer protocol called Phonenet by Farralon. Phonenet uses the Black and Yellow for Network communications. It is for use with Appletalk and is a replacement for Localtalk. It runs at the Localtalk speed of 230 Kbps which is reasonable for small networks.

i. Ringer Unit

The ringer is a device to alert you to an incoming call. It interprets the ringing voltage from the Central Office. Originally, the ringer was a electromagnetic bell but today, most ringers are electronic devices.

The Central Office sends:

a 90 to 120 VAC ringing voltage Frequency of 20 Hz

Cadence for North America is 2 sec On/ 4 sec Off

ii. Hook Switch

The hook switch is a switch that is activated by lifting the handset off the cradle. The position of the hook switch determines whether the telephone is waiting for a call or actively using the line. The Off-hook position informs the network of a request for use. The On-hook position releases the use of the network.

iii. Dialer Unit

There are two types of Dialer Units: Rotary Dial and Touch Tone. Rotary Dial are the old put your finger in the hole and spin type. The rotary dial operates by toggling the Hook Switch on and off.

Touch Tone is the modern method where 2 frequencies per push button are sent. Touch Tone is a trade name, the correct name is DTMF (Dual Tone Multi Frequency).

iv. Hybrid/Speech Network

The Hybrid/Speech Network performs several functions:

It converts the Tx/Rx 4 wires from the Handset to the 2 wires for the Local Loop.

It interfaces the signals from the Dialer Unit to the telephone line.

It provides auto line compensation for line length to keep the volume constant.

v. Handset

The Handset contains transducers for converting mechanical energy into electrical energy. The microphone converts speech into electrical energy. The diaphragm or speaker converts electrical signals into audible signals.

Functions of a Telephone Set:

i. Request use of network from the CO (Central Office).

ii. Inform you of the network status: Dial-tone, Ringing, Busy, Fast Busy (Talk Mail)

iii. Informs CO of desired number.

iv. Informs you when a call is incoming (phone rings).

v. Releases use of network when call is complete (hang-up)

vi. Transmit speech on network & receives speech from distant caller.

vii. Adjust power levels and compensates for line length.

24b. Local Loops

The Local Loop is the connection between the Central Office and the home or business. To every home is run 2 wires (1 pair). The pair does not go directly to the Central Office, instead it goes to those big green boxes called "Serving Area Interfaces" (SIA) that you see on the street corners. Then large multi-conductor bundles of wires go from there to the Central Office.

24c. Central Office

The Central Office provides the following functions:i. It supplies the battery voltage for the telephone system. The On-hook voltage is

48 Vdc +/- 2V. Off-hook voltage is -6.5 Vdc.ii. It supplies the Ringing Generator - 90 to 120 VAC, 20 Hz, 2 sec on/ 4 sec off

iii. It supplies the Busy signal (480 + 620 Hz, 0.5 sec On/ 0.5 sec Off), Dial Tone (350 + 440 Hz) and Fast Busy (480 + 620 Hz, 0.2 sec On/ 0.3 sec Off).

iv. It has the digital switching gear that determines if the number is an Interoffice call (local) or an Intraoffice call (Toll - long distance).

24d. Hierarchical Phone Networks

The PSTN (Public Switch Telephone Network) is divided into a hierarchical network. There are 5 classes of switching centres in North America:

Class Centre Abbreviation Symbol Examples

1 Regional Center RC 2 in Canada: West - ReginaEast - Montreal

2 Sectional Center SC Calgary serves Alberta

3 Primary Center PC Edmonton

4 Toll Center TC Drumheller

4b Toll Point TP Rainbow Lake

5 Central Office(Local Loop) CO 284-xxxx

In the following example:

The Hierarchical portion is seen as:

Trunk Long distance telephone cableToll Trunk Connects CO (Central Office) to TC (Toll Center)Intertoll Trunk Everything above TC (Toll Center) and TC to TCInteroffice Trunk Between CO (Central Office)Intraoffice Trunk Call between 2 subscribers within the same CO (284-7079 to 284-8181).

Call routing:

1. Preferred route2. Second choice

3. Third Choice

Call routing is determined by network engineering and physical location. When all lines are idle, the call routing selects the preferred route. If the preferred route is busy, then the call is routed to the second choice. Because the second choice is routed through one toll center, the charge for the call is greater than the preferred route. The third choice is used when the second choice is busy. The third choice goes through 2 toll centers and is the most expensive route.

A Central Office can have up to 10,000 subscribers: for example 284-0000 to 284-9999. Most have 4,000 to 5,000 subscribers. The Central Office bases the loading requirements on roughly 10% of the phones will be in use at any one time. The use of Internet dialup access has drastically changed this!

25. Telephone Line Characteristics

Telephone lines are not perfect devices due to their analog nature. The quality of the telephone line determines the rate that modulated data can be transferred. Good noise free lines allow faster transfer rates such as 14.4 kbps, poor quality lines require the data transfer rate to be stepped down to 9600 bps or less. Phone lines have several measurable characteristics that determine the quality of the line:

Attenuation Distortion Propagation Delay

Envelope Delay Distortion

25a. Attenuation Distortion

Attenuation Distortion is the change in amplitude of the transmitted signal over the Voice Band. It is the frequency response curve of the Voice Band.

Attenuation versus Frequency

To measure Attenuation Distortion, the phone line has a test frequency transmitted from 0 - 4 kHz into the line at a standard amplitude of 0 db. The loss of signal or attenuation is measured at the receiving end and compared to a standard reference frequency: 1004 Hz.

db is short for decibel which is a relative unit of measure (similar to a unit like a dozen). It is a log unit and a +3 db gain will indicate an amplitude of 2x the reference. It is a logarithmic ratio between input voltage and output voltage. It is calculated by the following formula:

db =10 x log (Vout/Vin)

The resulting information is graphed on an Attenuation vs. Frequency chart. Attenuation is a loss of signal amplitude - the receive signal is a smaller amplitude than the transmitted signal. It is indicated by a positive db. It is also possible to have a signal appear at the receiving end with a larger amplitude than when it started - this is indicated by negative db.

The attenuation is due to the many pieces of electronic equipment and transmission media that the signal has to pass through, some can amplify the signal (make it a larger amplitude) and some may attenuate the signal (make it smaller).

There are maximum and minimum acceptable limits for Attenuation Distortion for phone lines. The Basic channel conditioning is:

Frequency Range Loss (db)500 - 2500 -2 to +8300 - 3000 -3 to +12

The above Loss is a range of acceptable values for the frequency range. In the Basic Channelling Conditioning, it is acceptable to have a loss in signal in the frequency range of 500-2500 Hz of "8 db loss to -2 db loss" referenced to the amplitude at 1 kHz. Note that on the graph on the previous page that this is shown as -8db and +2 db.

+3 db attenuation is equal to -3 db in signal amplitude and +8 db attenuation equates to -8 db in signal amplitude.

25b. Propagation Delay

Signals transmitted down a phone line will take a finite time to reach the end of the line. The delay from the time the signal was transmitted to the time it was received is called Propagation Delay. If the propagation delay was the exact same across the frequency range, there would be no problem. This would imply that all frequencies from 300 to 3000 Hz have the same amount of delay in reaching their destination over the phone line. They would arrive at the destination at the same time but delayed by a small amount called the propagation delay.

This is heard as the delay when talking on long distance telephones. We have to wait a little longer before we speak to ensure that the other person hasn't already started to talk. All phone lines have propagation delay.

If the Propagation Delay is long enough, the modem or communications package may time-out and close the connection. It may think that the receive end has shut off!

25c. Envelope Delay Distortion

If the Propagation Delay changes with frequency than we would have the condition where the lower frequencies such as 300 Hz may arrive earlier or later than the higher frequencies such as 3000 Hz. For voice communication, this would probably not be noticable but for data communication using modems, this could affect the phase of the carrier or the modulation technique used to encode the data.

When the Propagation Delay varies across the frequency range, we call this Envelope Delay Distortion. We measure propagation delay in microseconds (us) and the reference is from the worst case to the best case.

6. Line Impairments

Line Impairments are faults in the line due to improper line terminations or equipment out of specifications. These cannot be conditioned out but can be measured to determine the amount of the impairment.

26a. Crosstalk

Crosstalk is when one line induces a signal into another line. In voice communications, we often hear this as another conversation going on in the background. In digital communication, this can cause severe disruption of the data transfer. Cross talk can be caused by overlapping of bands in a multiplexed system or by poor shielding of cables running close to one another. There are no specific communications standards applied to the measurement of crosstalk.

26b. Echo or Signal Return

All media have a preferred termination condition for perfect transfer of signal power. The signal arriving at the end of a transmission line should be fully absorbed otherwise it will be reflected back down the line to the sender and appear as an Echo. Echo Suppressors are often fitted to transmission lines to reduce this effect.

Normally during data transmission, these suppressors must be disabled or they will prevent return communication in full duplex mode. Echo suppressors are disabled on the phone line if they hear carrier for 400ms or more. If the carrier is absent for 100 mSec, the echo suppressor is re-enabled.

Echo Cancellers are currently used in Modems to replicate the echo path response and then combine the results to eliminate the echo. Thus no signal interruption is necessary.

26c. Frequency Shift

Frequency shift is the difference between the transmitted frequency and the received frequency. This is caused by the lack of synchronization of the carrier oscillators.

26d. Non-Linear Distortion

Non-linear distortion is distortion that changes the waveshape of the signal. If the signal was transmitted as a sinewave and arrived as a squarewave, this would be an example of severe non-linear distortion. Amplitude modulated carriers would suffer drastically if the original wave shape was distorted.

26e. Jitter: Amplitude and Phase

There are 2 types of Jitter:a. Amplitude Jitterb. Phase Jitter

Amplitude Jitter is the small constantly changing swings in the amplitude of a signal. It is principally caused by power supply noise (60 Hz) and ringing tone (20 Hz) on the signal.

Phase Jitter is the small constantly changing swings in the phase of a signal. It may result in the pulses moving into time slots allocated other data pulses when used with Time Domain Multiplexing.

Telephone company standards call for no more than 10 degrees between 20 and 300 Hz and no more than 15 degrees between 4 and 20 Hz.

26f. Transients: Impulse Noise, Gain Hits, Dropouts & Phase Hits

Transients are irregular timed impairments. They appear randomly and are very difficult to troubleshoot. There are 4 basic types of Transients:

i. Impulse Noiseii. Gain Hits

iii. Dropouts

iv. Phase Hits

i. Impulse Noise

Impulse noise is sharp quick spikes on the signal caused from electromagnetic interference, lightning, sudden power switching, electromechanical switching, etc.. These appear on the telephone line as clicks and pops which are not a problem for voice communication but can appear as a loss of data or even as wrong data bits during data transfers. Impulse noise has a duration of less than 1 mSec and their effect is dissipated within 4 mSec.

ii. Gain Hits

Gain Hits are sudden increase in amplitude that last more than 4 mSec. Telephone company standards allow for no more than 8 gain hits in any 15 minute interval. A gain hit would be heard on a voice conversation as if the volume were turned up for just an instance. Amplitude modulated carriers are particularly sensitive to Gain Hits.

iii. Dropouts

Dropouts are sudden loss of signal amplitude greater than 12 db that last longer than 4 mSec. They cause more errors than any other type of transients. Telephone company standards allow no more than 1 dropout for every 30 minute interval. Dropouts would be heard on a voice conversation similar to call waiting, where the line goes dead for a 1/2 second. This is a sufficient loss of signal for some digital transfer protocols such as SLIP, that the connection is lost and would have to be re-established.

iv. Phase Hits

Phase Hits are sudden large changes in the received signal phase (20 degrees) or frequency lasting longer than 4 mSec. Phase Hits generally occur when switching between Telcos, common carriers or transmitters. FSK and PSK are particularly sensitive to Phase Hits. The data may be incorrect until the out of phase condition is rectified. The telephone company standard allows no more than 8 phase hits in any 15 minutes.

27. Modulation TechniquesModulation techniques are methods used to encode digital information in an analog world. The 3 basic modulation techniques are:

a. AM (amplitude modulation)b. FM (frequency modulation)

c. PM (phase modulation)

All 3 modulation techniques employ a carrier signal. A carrier signal is a single frequency that is used to carry the intelligence (data). For digital, the intelligence is either a 1 or 0. When we modulate the carrier , we are changing its characteristics to correspond to either a 1 or 0.

27a. AM - Amplitude Modulation

Amplitude Modulation modifies the amplitude of the carrier to represent 1s or 0s. In the above example, a 1 is represented by the presence of the carrier for a predefined period of 3 cycles of carrier. Absence or no carrier indicates a 0.

Advantages:

Simple to design.

Disadvantages:

Noise spikes on transmission medium interfere with the carrier signal. Loss of connection is read as 0s.

27b. FM - Frequency Modulation

Frequency Modulation modifies the frequency of the carrier to represent the 1s or 0s. In the above example, a 0 is represented by the original carrier frequency and a 1 by a much higher frequency ( the cycles are spaced closer together).

Advantages:

Immunity to noise on transmission medium. Always a signal present. Loss of signal easily detected

Disadvantages:

Requires 2 frequencies Detection circuit needs to recognize both frequencies when signal is lost.

27c. PM - Phase Modulation

Phase Modulation modifies the phase of the carrier to represent a 1 or 0.

The carrier phase is switched at every occurrence of a 1 bit but remains unaffected for a 0 bit. The phase of the signal is measured relative to the phase of the preceding bit. The bits are timed to coincide with a specific number of carrier cycles (3 in this example = 1 bit).

Advantage:

Only 1 frequency used Easy to detect loss of carrier

Disadvantages:

Complex circuitry required to generate and detect phase changes.

28. Modem ModulationThere are 3 basic types of modulation used in modems:

a. FSK - Frequency Shifted Keyingb. QPSK - Quadrature Phase Shifted Keying

c. QAM - Quadrature Amplitude Modulation

Modern modems use a combination of the above basic modulation techniques and compression to achieve the high data transfer rates (14.4 Kbps and up).

28a. FSK - Frequency Shift Keying

Frequency Shift Keying or FSK is the frequency modulation of a carrier to represent digital intelligence. For Simplex or Half Duplex operation, a single carrier (1170 Hz) is used - communication can only be transmitted in one direction at a time. A Mark or 1 is represented by 1270 Hz, and a Space or 0 is represented by 1070 Hz. The following diagram shows the Voice Channel with Simplex/Half Duplex FSK.

Simplex/Half Duplex FSK

Full Duplex FSK

For Full Duplex, (data communication in both directions simultaneously) the upper bandwidth of the Voice Channel is utilized. Another carrier is added at 2125 Hz. A Mark or 1 is represented by 2225 Hz, and a Space or 0 is represented by 2025 Hz. The originating modem (the one which dials the phone number and starts the connection) uses the lower carrier (1170 Hz) and the answer modem (the one which answers the ringing phone line) uses the upper carrier (2125 Hz). This allocation of carriers is done automatically by the modem's hardware. The following diagram shows the Voice Channel with Full Duplex FSK.

Example of Originate's Frequency Modulated Carrier:

The originate modem transmits on the 1170 Hz carrier and receives on the 2125 Hz carrier. The answer modem receives on the 1170 Hz carrier and transmits on the 2125 Hz carrier. This way both modems can be transmitting and receiving simultaneously!

The FSK modem described above is used for 300 baud modems only. The logical question is "Why not use it for higher modems?". Higher data rates require more bandwidth: this would require that the Mark and Space frequencies for each band be moved farther apart (the originate and answer bands become wider). The two carriers would have to move farther apart from each other to prevent crosstalk (interference

with each other). The limit for present phone lines is 1200 Baud Half Duplex (one way) used by Bell 202 compatible modems.

28b. QPSK - Quadrature Phase Shift Keying

Quadrature Phase Shift Keying employs shifting the phase of the carrier at a 600 baud rate plus an encoding technique. QPSK is used in Bell 212A compatible modems and V.22 - both are 1200 bps Full Duplex standards. The originate modem transmits at 1200 Hz and receives on 2400 Hz. The answer modem receives on 1200 Hz and transmits on 2400 Hz.

The digital information is encoded using 4 (Quad) level differential PSK at 600 baud.

Remember that baud indicates how fast the analog signal is changing in the Voice Channel. The data is encoded as follows:

DIBIT Phase Shift00 +9001 010 18011 270

For every change in the baud rate (phase shift), we can decode 2 bits! This leads to:

2 bits x 600 baud = 1200 bps

Example of Carrier Phase Modulation:

28c. QAM - Quadrature Amplitude Modulation

Quadrature Amplitude Modulation refers to QPSK with Amplitude Modulation. Basically, it is a mix of phase modulation and amplitude modulation. QAM phase modulates the carrier and also modulates the amplitude of the carrier.

Phase Modulated and Amplitude Modulated Carrier:

There are two types: 8-QAM and 16-QAM. 8-QAM encodes 3 bits of data (23=8) for every baud and 16-QAM encodes 4 bits of data (24=16) for every baud. Both are used in the V.32 standard for 9600 bps modem (milestone for communications!). 8-QAM transfers 4800 bps and 16-QAM transfers 9600 bps. The baud rate used with QAM is 2400 baud half-duplex.

16-QAM has 12 phase angles, 4 of which have 2 amplitude values! 16-QAM changes phase with every baud change.

16-QAM Phasor Diagram

Higher transfer rates use much more complex QAM methods. For example, V.32bis (14.4 kbps) uses a 64 point constellation to transfer 6 bits per baud. Compare that to the above 16 point constellation!

29. AT Command SetHayes modems were the first smart modems. They had built-in CPUs that could interpret a special series of commands. These commands are called the AT command set. The basic command for getting a modem's attention was the characters "AT" (older modems may only recognize lower case "at"). Once the modem's attention was available, character's are added immediately after that specify instructions.

Smart modems operate in two modes: command and communication mode. In command mode, the modem is waiting for AT command instructions. In communication mode, the modem is transferring data from sender to receiver.

To talk to a modem, you must use either a terminal or a terminal emulation software on a PC such as Procomm or Hyperterminal. A basic test to see if the modem is communicating properly with the terminal, is to type "AT". If the modem responds with "OK", then the software's configuration matches the modems configuration.

The following configuration issues must match before proper modem to terminal communication will work:

Configuration Point Typical valueCom port of modem: Com2 for external, Com4 for internalIRQ of modem: IRQ3Number of data bits: 8Type of parity: n (none)Number of stop bits: 1Transfer speed: 56 kbps (depends on modem)Terminal emulation: vt100

If the modem is on-line (communicaton mode), to enter command mode, type "+++" (3 pluses in a row) and wait. The modem should respond with "OK". This indicates that you have entered command mode. You then may enter AT command strings to the modem.

29a. Basic AT commands

Modern modems require an initialization string for configuring themselves. The most common configuration string is "ATZ", which is the reset command. Usually and this will depend on the modem, there are factory stored configurations that can be accessed by using the "ATF1" command. If there are more than one available configuration, the others can be accessed by "ATF2" and so on.

Older modems, typically 14.4kbps and earlier, had elaborate initialization strings that differed for each modem and each manufacturer. It was quite a headache to support so many different types of modems and make them work with each other.

To dial out, the "ATD" command is used. "ATDT" uses tone dialing versus rotary dialing. Immediately after the "ATDT" command, the destination telephone number is entered, for example: "ATDT555-1234". Would command the modem to use tone dialing to dial the number 555-1234. To hang up a modem, the AT command string "ATH" can be used.

A partial listing of the AT command set is available in Appendix J. The AT command set is incredibly large and is constantly growing due to the improvements and innovations by the modem manufacturers. There are two main manufacturer's of modem chipsets: Rockwell and US Robotics. Both have excellent documentation on identifying and configuring the modem chipsets that they manufacture.

30. MultiplexingMultiplexing is the transmission of multiple data communication sessions over a common wire or medium. Multiplexing reduces the number of wires or cable required to connect multiple sessions. A session is considered to be data communication between two devices: computer to computer, terminal to computer, etc..

Individual lines running from 3 terminals to one mainframe is not a problem but when the number of terminals increases to 10 and up, it becomes a problem. Imagine a mainframe computer with 1200 terminals connected and each terminal running its own wire to the mainframe. If each wire was 1/4" in diameter (typical Cat 5 cable), you would have a wiring bundle going into the computer, roughly 2 feet in diameter.

A multiplexer allows sharing of a common line to transmit the many terminal communications as in the above example. The connection between the multiplexer and the mainframe is normally a high speed data link and is not usually divided into separate lines.

The operation of multiplexers (abbreviated MUXs) is transparent to the sending and receiving computers or terminals. Transparent means that as far as everyone is concerned, they appear to be directly connected to the mainframe with individual wires. The multiplexer does not interfere with the normal flow of data and it can allow a significant reduction in the overall cost of connecting to remote sites, through the reduced cost of cable and telephone line charges.

Multiplexers are used to connect terminals located throughout a building to a central mainframe. They are also used to connect terminals located at remote locations to a central mainframe through the phone lines.

There are 3 basic techniques used for multiplexing:

a. Frequency Division Multiplexing (FDM)b. Time Division Multiplexing (TDM)

c. Statistical Time Division Multiplexing (STDM)

d. 30a. FDM - Frequency Division Multiplexinge. Frequency Division Multiplexing (FDM) is an analog technique where each

communications channel is assigned a carrier frequency. To separate the channels, a guard-band would be used. This is to ensure that the channels do not interfere with each other.

f. For example, if we had our 3 terminals each requiring a bandwidth of 3 kHz and a 300 Hz guard-band, Terminal 1 would be assigned the lowest frequency channel 0 - 3 kHz, Terminal 2 would be assigned the next frequency channel 3.3 kHz - 6.3 kHz and Terminal 3 would be assigned the final frequency channel 6.6 kHz - 9.6 kHz.

g. The frequencies are stacked on top of each other and many frequencies can be sent at once. The downside is that the overall line bandwidth increases. Individual terminal requirement were 3 kHz bandwidth each, in the above example: the bandwidth to transmit all 3 terminals is now 9.6 kHz.

h.

i. FDM does not require all channels to terminate at a single location. Channels can be extracted using a multi-drop technique, terminals can be stationed at different locations within a building or a city.

j. FDM is an analog and slightly historical multiplexing technique. It is prone to noise problems and has been overtaken by Time Division Multiplexing which is better suited for digital data.

k.

l. 30b. TDM - Time Division Multiplexingm. Time Division Multiplexing is a technique where a short time sample of each

channel is inserted into the multiplexed data stream. Each channel is sampled in

turn and then the sequence is repeated. The sample period has to be fast enough to sample each channel according to the Nyquist Theory (2x highest frequency) and to be able to sample all the other channels within that same time period. It can be thought of as a very fast mechanical switch, selecting each channel for a very short time then going on to the next channel.

n.

o. Each channel has a time slice assigned to it whether the terminal is being used or not. Again, to the send and receiving stations, it appears as if there is a single line connecting them. All lines originate in one location and end in one location. TDM is more efficient, easier to operate, less complex and less expensive than FDM.

p.

30c. STDM - Statistical Time Division Multiplexing

Statistical Time Division Multiplexing uses intelligent devices capable of identifying when a terminal is idle. They allocate time only to lines when required. This means that more lines can be connected to a transmission medium as this device statistically compensates for normal idle time in data communication lines. Newer STDM units provide additional capabilities such as data compression, line priority, mixed speed lines, host port sharing, network port control, automatic speed detection and much more.

31. Telecommunication MultiplexingTelecommunication multiplexing is used between switching offices on Interoffice trunks and Intertoll trunks. The Telcos (telecommunication companies such as Bell Canada, AGT, BC-Tel, etc..) share communication facilities which can be either FDM or TDM. A communication path can change in mid-stream from FDM to TDM and back again depending on where or whose communication facility is being used.

q.

r. FDM is analog and is being updated to TDM throughout the world. Still today, there are locations where FDM is being used.

31a. FDM - Channel GroupsTelecommunications FDM is based on channel groups. The basic channel is called the Voice Channel and it has a bandwidth of 0-4 kHz. The channel groups are based on multiples of the voice channel:

Multiplex Level Voice Circuits Freq Band(kHz)

BW(kHz)

Voice Channel 1 0 - 4 4Group 12 60 - 108 48Supergroup 60 312 - 552 240

Mastergroup 600 564 - 3,084 2520Jumbogroup 3600 564 - 17,548 16984

The Mastergroup and Jumbogroup have guard-bands added to the bandwidth. A Group is made of 12 Voice Channels.

A Supergroup (60 Voice channels) is made of 5 Groups (12 Voice Channels).

A Mastergroup (600 Voice Channels) is made of 10 Supergroups (60 Voice Channels).

A Jumbogroup (3600 Voice Channels) is made of 6 Mastergroups (600 Voice Channels).

31b. TDM - T1 Carrier System

Telecommunications TDM is based on the T1 Carrier System. It is a digital system that digitizes the analog Voice Channel into 8 bit data. This means that there are 28 or 256 levels that the 8 bit data can represent.

It samples the analog signal 8000 times a second (2x 4 kHz - makes Nyquist happy!). It is a serial data stream so we transmit the 8 bit data 1 bit at a time. This means that for a digitized Voice Channel, the data rate is:

8 bits x 8000 samples = 64 Kbps

The basic Carrier used in the T1 Carrier System is called the T1 (sometimes called DS-1) and it carries 24 Voice Channels. The Bit Rate for the T1 Carrier is 1.544 Mbps. If we multiply:

24 Voice Channels x 64 Kbps per Voice Channel = 1.536 Mbps

The missing 8 K is used to "frame" the data. It is information used for the Start Frame bytes, End Frame, Error Checking, Routing information, etc..

Digital Circuit Voice Channels Bit Rate # of T1 CircuitsT1 (DS-1) 24 1.544 Mbps 1T2 (DS-2) 96 6.312 Mbps 4T3 (DS-3) 672 44.736 Mbps 28T4 (DS-4) 4032 274.176 Mbps 168

Typically:

T1 - Twisted Pair or Coax Cable T2 - Coax Cable

T3 - Coax, Fibre Optics or Light Route Radio

T4 - Coax or Fibre Optics

You can rent any quantity of a T1 line, you don't have to rent the complete circuit. You basically rent a time-slot on the line based on 64 kbps channels. This is called Fractional T-1.

32. Introduction to the ISO - OSI ModelThe ISO (International Standards Organization) has created a layered model called the OSI (Open Systems Interconnect) model to describe defined layers in a network operating system. The purpose of the layers is to provide clearly defined functions to improve internetwork connectivity between "computer" manufacturing companies. Each layer has a standard defined input and a standard defined output.

Understanding the function of each layer is instrumental in understanding data communication within networks whether Local, Metropolitan or Wide.

32a. OSI Model Explained

This is a top-down explanation of the OSI Model, starting with the user's PC and what happens to the user's file as it passes though the different OSI Model layers. The top-down approach was selected specifically (as opposed to starting at the Physical Layer and working up to the Application Layer) for ease of understanding of how the user's files are transformed through the layers into a bit stream for transmission on the network.

There are 7 Layers of the OSI model:

7. Application Layer (Top Layer) 6. Presentation Layer

5. Session Layer

4. Transport Layer

3. Network Layer

2. Data Link Layer

1. Physical Layer (Bottom Layer)

 

32b. Layer 7 - Application Layer

Fig. 1 Basic PC Logical Flowchart

A basic PC logical flowchart is shown in Fig. 1. The Keyboard & Application are shown as inputs to the CPU that would request access to the hard-drive. The Keyboard requests accesses to the hard-drive through user enquiries such as "DIR" commands and the Application through "File Openings" and "Saves". The CPU, through the Disk Operating System, sends/receives data from the local hard-drive ("C:" in this example).

A PC setup as a network workstation has a software "Network Redirector" (actual name depends on the network - we will use a generic term) placed between the CPU and DOS as in Fig 2. The Network Redirector is a TSR (Terminate and Stay Resident) program which presents the network hard-drive as another local hard-drive ("G:" in this example) to the CPU. Any CPU requests are intercepted by the "Network Redirector". The Network Redirector checks to see if a local drive is requested or a network drive. If a local drive is requested, the request is passed on to DOS. If a network drive is requested, the request is passed on to the network operating system (NOS).

Electronic mail (E-Mail), client-server databases, games played over the network, print and file servers, remote logons and network management programs or any "network aware" application are aware of the network redirector and can communicate directly with other "network applications" on the network. The "Network Aware Applications" and the "Network Redirector" make up Layer 7 - the Application layer of the OSI Model as shown in Fig 3.

Fig. 2 Simple Network Redirection

 

Fig. 3 PC Workstation with Network Aware Software

32c. Layer 6 - Presentation Layer

The Network Redirector directs CPU operating system native code to the network operating system. The coding and format of the data is not recognizable by the network operating system. The data consists of file transfers and network calls by network aware programs.

As an example: when a dumb terminal is used as a workstation in a mainframe or minicomputer network, the network data is translated into and from the format that the terminal can use. The Presentation layer presents data to and from the terminal using special control characters to control the screen display (LF-linefeed, CR-carriage return, cursor movement, etc..). The presentation of data on the screen would depend on the type of terminal VT100, VT52, VT420, etc.

Similarly, the Presentation layer strips the pertinent file from the workstation operating system's file envelope. The control characters, screen formatting and workstation operating system envelope are stripped or added to the file, depending on if the workstation is receiving or transmitting data to the network. This could also include translating ASCII files characters from a PC world to EBCDIC in an IBM Mainframe world.

The Presentation Layer also controls security at the file level. This provides file locking and user security. The DOS Share program is often used for file locking. When a file is in use, it is locked from other users to prevent 2 copies of the same file to be generated. If 2 users both modified the same file and User A saved it then User B saved it - User A's changes would be erased!

At this point, the data is contiguous and complete at this point (one large data file). See Fig. 4.

32d. Layer 5 - Session Layer

The Session layer manages the communications between the workstation and network. The Session layer directs the information to the correct destination and identifies the source to the destination. The Session layer identifies the type of information as data or control. The Session layer manages the initial start-up of a

session and the orderly closing of a session. The Session layer also manages Logon procedures and Password recognition. See Fig. 5.

Fig. 5 Session Layer

32e. Layer 4 - Transport Layer

In order for the data to be sent across the network, the file must be broken up into usable small data segments (typically 512 - 18K bytes). The Transport layer breaks up the file into segments for transport to the network and combines incoming segments into a contiguous file. The Transport layer does this logically not physically, it is done in software as opposed to hardware.

The Transport layer provides error checking at the segment level (frame control sequence). This checks that the datagrams are in the correct order and the Transport layer will correct out of order datagrams. The Transport layer guarantees an error-free host to host connection, it is not concerned with the path between machines.

32f. Layer 3 - Network Layer

The Network layer is concerned about the path through the network. It is responsible for routing, switching and controlling the flow of information between hosts. The Network layer converts the segments into smaller datagrams that the network can

handle. The Network layer does not guarantee that the datagram will reach its destination. The network hardware source and destination addresses are added.

Fig. 7 Network Layer

32g. Layer 2 - Data Link Layer

The Data Link layer is a firmware layer of the network interface card. The Data Link layer puts the datagrams into packets (frames of bits: 1s & 0s) for transmission and assembles received packets into datagrams. The Data Link layer works at the bit level and adds start/stop flags and bit error checking (CRC or parity) to the packet frame. Error checking is at the bit level only, packets with errors are discarded and a request for re-transmission is sent out. The Data Link layer is concerned about bit sequence.

Fig. 8 Data Link Layer

32h. Layer 1 - Physical Layer

The Physical layer concerns itself with the transmission of bits and the network card's hardware interface to the network. The hardware interface involves the type of cabling

(coax, twisted pair, etc..), frequency of operation (1 Mbps, 10Mbps, etc..), voltage levels, cable terminations, topography (star, bus, ring, etc..), etc.. Examples of Physical layer protocols are 10Base5 - Thicknet, 10Base2 - Thinnet, 10BaseT - twisted pair, ArcNet, FDDI, etc.. See Fig. 9.

Fig. 9 Physical Layer

32i. Layer Specific Communication

Each layer may add a Header and a Trailer to its Data which consists of the next higher layer's Header, Trailer and Data as it moves through the layers. The Headers contain information that addresses layer to layer communication specifically. For example: The Transport Header (TH) contains information that only the Transport layer sees and all other layers below the Transport layer pass the Transport Header as part of their Data.

PDU - Protocol Data Unit (fancy name for Layer Frame)

32j. OSI Model Functional Drawing

33. Synchronous TransmissionMessage Frames

Synchronous Transmission sends packets of characters at a time. Each packet is preceded by a Start Frame which is used to tell the receiving station that a new packet of characters is arriving and to synchronize the receiving station's internal clock. The packets also have End Frames to indicate the end of the packet. The packet can contain up to 64,000 bits depending on the protocol. Both Start and End Frames have a special bit sequence that the receiving station recognizes to indicate the start and end of a packet. The Start and End Frames may be only 2 bytes each.

Efficiency

Synchronous transmission is more efficient than asynchronous (character transmission) as little as only 4 bytes (2 Start Framing Bytes and 2 Stop Framing bytes) are required to transmit up to 8K bytes. Extra bytes, like the Start and Stop Frame, that are not part of the data are called overhead. Packet overhead consists of control information used to control the communication.

Efficiency example: An Ethernet frame has an overhead of 26 bytes including the "Start and Stop Frames", the maximum data size is 1500 bytes. What is the Ethernet frame's efficiency?

33a. Clocking: Self & Manchester Encoding

Synchronous transmission is more difficult and expensive to implement than asynchronous transmission. It is used with all higher transfer rates of communication: Ethernet, ArcNet, Token Ring etc... Synchronous transmission is used in fast transfer rates 100 Kbps to 100 Mbps. In order to achieve the high data rates, Manchester Line Encoding is used.

In the Manchester Code, there is a transition at the middle of each bit period. The mid-bit transition serves as a clocking mechanism and also as data: a low to high transition represents a 1 and a high to low transition represents a 0.

Manchester Encoding has no DC component and there is always a transition available for synchronizing receive and transmit clocks. Because of the continuous presence of these transitions, Manchester Encoding is also called a self clocking code.

It has the added benefit of requiring the least amount of bandwidth compared to the other Line Codes (Unipolar, Polar, etc..). Manchester coding requires 2 frequencies: the base carrier and 2 x the carrier frequency. All other types of Line Coding require a range from 0 hertz to the maximum transfer rate frequency. In other words, Manchester Encoding requires a Narrow Bandwidth

34. Basic Frame StructureThe Generic Packet X is used as an introduction to Synchronous Data Transmission. As we explore more standards and protocols, we find that we can expand the frame structure (packet) into better defined sections that will allow easier understanding of

different frame types (Ethernet, Token Ring, SDLC, HDLC, Frame Relay, ATM, Cell Relay, etc...). It also will provide a point of reference.

Basic Frame Structure

34a. Preamble: Starting Delimiter/Alert Burst/Start of Header

At the beginning of each frame (packet), there will be a sequence of octets (8 bit words), called the Preamble. The Preamble is used to:

Inform the receiving station that a new packet is arriving Synchronize the receive clock with the transmitted clock

The Preamble is a series of octets with a specific bit pattern that is used only by the Preamble.

Names used by other protocols for the Preamble are: Starting Delimiter, Alert Burst and Start of Header. All perform the same 2 basic functions.

34b. Address Field(s): Source and/or Destination

The Address Field consists of a Source Address and/or a Destination Address. The Source and Destination Addresses are hexadecimal numbers that identify the sender - Source and receiver - Destination. The Network Addresses reside in either the Network Interface Card's firmware or can be either assigned during the initialization of the NIC.

The purpose of the Source Address is to identify to the network who is sending data. The purpose of the Destination Address is to identify to the network who should be receiving the data.

Under some protocols, there may not be both Source and Destination Addresses. Only one address may be present.

34c. Control Field

The Control Field is used to indicate the Type of Information being sent as Data. The Type of Information can be Control information used when establishing a connection (handshaking) or it can be Data such as file transfers between clients and servers. The purpose of the Control Field is to identify what the purpose of the packet or frame is: Control or Data. It can also be used to indicate the size of the packet and Data.

34d. Data/Message and optional Pad

The Data Field or Message is the actual information that is being transmitted. It can contain Control Information for handshaking or actual Data used by applications. The Control Field would indicate the Data Field size. The Data field is also called the Info field by some protocols.

The optional Pad is used to pad the data field when the protocol has a fixed Data Field size. If the Data Field size is fixed at 1200 octets and only 300 octets of information is available then the Pad will fill in the remaining 900 octets with characters (e.g. 900 octets of 00h). The protocol may also use the Pad to ensure a minimum Data field size.

34e. CRC/ Frame Check Sequence

The CRC / Frame Check Sequence (FCS) contains an error checking number that the Destination can use to verify that the packet is okay and error-free. CRC is an abbreviation for Cyclic Redundancy Checking. The Frame Check Sequence typically incorporates a 32 Bit CRC check. Checksums work similarly but use a different algorithm.

As each packet is sent, the Source calculates a check number from the data using a predetermined algorithm (formula). The result of this calculation is appended to the packet in the Frame Check Sequence (FCS) field. At the Destination, the same calculation is performed and the result is compared to the transmitted Frame Check Sequence. If the result generated at the Destination is identical to the FCS, then it is assumed that the packet is error free at the bit level.

34f. End Frame Delimiter

The End Frame Delimiter is a series of octets that have a specific bit pattern that identifies the end of the packet to the Destination. Not all protocols have End Frame Delimiters fields, protocols with fixed packet size may not need the End Frame Delimiter field as the Destination may simply count the number of octets it has received.

35. Physical LayerThe OSI Model Physical Layer concerns itself with the transmission of bits through the communication medium. The order of the bits and importance is determined by the Protocol's packet.

35a. Asynchronous & Synchronous Communication

In Asynchronous Communications, the OSI Physical layer concerned itself with the RS-232D standard and the Voice Channel. The RS-232D standard stated the electrical and mechanical characteristics of the cable for the transmission of the digital signal between the DTE (PC) and DCE (modem). The Voice Channel stated the electrical and mechanical characteristics of the connection between DCE to DCE (modem to modem) through the phone lines.

The order of the bits was determined by the ASCII characters, the parity (Odd/Even/None), number of Stop Bits and the Transfer Protocol used. Examples of Transfer Protocols are:

Kermit Xmodem

Ymodem

Zmodem

Similarly, in Synchronous Communications, the electrical and mechanical characteristics of the cable for the transmission of the signal are defined by the protocol used between Network Interface Cards.

The electrical characteristics associated with the OSI Model's Physical layer are:

Transmission rate (bits/sec) Voltage levels

Line Encoding

Propagation delay

Termination impedance

The mechanical characteristics associated with the OSI Model's Physical layer are:

Connector type Cable type & size

Cable Length

Topology

Shielding

In summary, the OSI Physical Layer is concerned with the transmission of bits on the network: the order of bits, bit level error-checking, and the electrical & mechanical characteristics.

36. IEEE-802.3 ProtocolThe IEEE-802.3 Protocol is based on the Xerox Network Standard (XNS) called Ethernet. The IEEE-802.3 Protocol is commonly called Ethernet but it is just 1 version. There are 4 versions or flavours of the Ethernet frame:Ethernet_802.2 Frame type used on Netware 3.12 & 4.01Ethernet_802.3 Frame type used on Netware 3.x & 2.x (raw)Ethernet_II Frame type used on DEC, TCP/IPEthernet_SNAP Frame type used on Appletalk (SubNet Access Protocol)

NOTE: The Source and Destination must have the same Ethernet Frame type in order to communicate.

36a. CSMA/CD (Carrier Sense Multiple Access/ Collision Detect)

Bus arbitration is performed on all versions of Ethernet using the CSMA/CD (Carrier Sense Multiple Access/ Collision Detect) protocol. Bus arbitration is another way of saying how to control who is allowed to talk on the (medium) and when. Put simply, it is used to determine who's turn it is to talk.

In CSMA/CD, all stations, on the same segment of cable, listen for the carrier signal. If they hear the carrier, then they know that someone else it talking on the wire. If they don't hear carrier then they know that they can talk. This is called the Carrier Sense portion of CSMA/CD.

All stations share the same segment of cable and can talk on it similar to a party line. This is the Multiple Access portion of CSMA/CD.

If 2 stations should attempt to talk at the same time, a collision is detected and both stations back off for a random amount of time and then try again. This is the Collision Detect portion of CSMA/CD.

36b. IEEE 802.3 Ethernet Media Types

IEEE 802.3 defines 5 media types of IEEE 802.3 Ethernet Types:IEEE 802.3 10Base5 Thick Coax 10Mbps Baseband 500mIEEE 802.3a 10Base2 Thin Coax 10Mbps Baseband 185mIEEE803b 10Broad36 Broadband 10 Mbps Broadband 3600mIEEE802.3e 1Base5 StarLAN 1 Mbps Baseband 500mIEEE 802.3i 10BaseT Twisted Pair 10Mps Baseband 100m

IEEE 802.3 - 10Base5 (Thick Coax) is used only as backbones to networks. Backbones are lines that connect buildings & network equipment together such as Bridges, Routers, Brouter, Hubs, Concentrators, Gateways, etc.. 10Base5 is being replaced by either Thin Coax or fibre optics.

IEEE 802.3a - 10Base2 is commonly used in new installations as a backbone to connect buildings and network equipment together. 10Base2 (Thin Coax) is also used to connect work-stations together but the preferred choice is to use 10BaseT.

IEEE 802.3b - 10Broad36 is rarely used, it combined analog and digital signals together. Broadband means that a mixture of signals can be sent on the same medium.

IEEE 802.3e - StarLAN is a slow 1 Mbps standard that has been replaced by Thin Coax or Twisted Pair.

IEEE 802.3i - 10BaseT is commonly used to connect workstations to network hubs. The network hubs can use 10BaseT (Twisted Pair) to connect to other Hubs.

36c. IEEE 802.3 10Base5

10Base5 Specifications :

Coaxial Cable

Uses double shielded 0.4 inch diameter RG8 coaxial cable about the size of a garden hose. The cable is not flexible and difficult to work with. The cable has a characteristic impedance of 50 ohms.

Connection to the workstation is made with a MAU - Medium Attachment Unit or Transceiver. The MAU physically and electrically attaches to the coaxial cable by a cable tap. The cable is pierced and a connection is made by a screw to the center conductor.

The MAU is connected to the NIC (Network Interface Card) by the AUI (Attachment Unit Interface) cable. The AUI port on a NIC and a MAU is a DB15 connector. Maximum AUI cable length is 50 m.

Cable Termination and Connector

The standard termination is 50 +/-2 ohms. The end connector on the RG-8 cable is an "N" type connector. The cable is externally terminated with a resistor inside an N connector.

Grounding

To minimize noise on the segment, the cable is grounded at the termination at only one end.

Maximum Nodes on a cable segment

On any 1 cable segment, the maximum allowed number of nodes or MAUs is 100.

Minimum Distance between nodes

Minimum distance between nodes or MAUs is 2.5 m or 8 feet.

Velocity of propagation

The speed of the signal through the cable is 0.77c. ("c" is equal to the speed of light - 300,000,000 m/sec). The velocity of propagation for 10Base5 specification cable is equal to 0.77 x 300,000,000 m/sec. This is determined by cable capacitance. Maximum coaxial cable segment length 500 m

The maximum segment length is 500 m or a maximum 2.165 uSec propagation delay. Propagation delay is what actually determines the maximum length of the segment.

Propagation delay for a specific cable length in meters is calculated by:

What is the propagation delay for a 500 m length of 10Base5 cable?

Maximum Number of Segments

Maximum of 5 segments (with 4 repeaters) can be along the path between any 2 network nodes: 3 may be coax segments having a maximum delay of 2.165 uSec and 2 may be link segments having a maximum delay of 2.570 uSec.

With no link segments used 3 populated coax segments can exist on a path.

5-4-3 Rule

The 5-4-3 Rule states that you are allowed 5 segments with 4 repeaters and 3 populated segments.

Maximum Transfer Rate

The Maximum Data Transfer Rate for IEEE 802.3 is 10 Mbps (10,000,000 bits per second of data). In actual fact, data transfer is dependant on how many users are fighting for the bus and how fast the user's data can get on the bus.

Physical Bus/Logical Bus

IEEE 802.3 is a Physical Bus - the cable is physically laid out as 1 long cable with the network nodes attached to it. It is also treated as a Logical Bus - electronically and logically it appears as 1 long cable with the network nodes attached to it.

36d. IEEE 802.3a 10Base2

Coaxial Cable

Uses RG-58A/U coaxial cable, 0.2 inch in diameter. The cable is flexible and easy to work with. The cable has a characteristic impedance of 50 ohms.

Connection to the workstation is made with either a MAU - Medium Attachment Unit/Transceiver or directly to the NIC using a BNC TEE.

Most NICs have the MAU built-in for 10Base2. The 3C509 card in the lab have built-in MAUs for Coax (10Base2) and Twisted Pair (10BaseT). They also have a AUI connection for an external MAU such as used in 10Base5. You can buy MAUs for 10Base2 and 10BaseT if your NIC does not have them already built-in.

Cable Termination and Connector

The standard termination is 50 +/-2 ohms. The end connector is an "BNC" twist and lock type connector. The cable is externally terminated with a special terminating BNC connector. BNC stands for Bayonet Navy Connector.

Grounding

To minimize noise on the segment, the cable is floating. The IEEE 802.3a specifications calls for all BNC connectors and TEEs to be insulated. A common problem with 10Base2 is having the barrel of the BNC connector touching a heating duct or computer chassis. The shield should be floating, it is not connected to electrical ground.

Maximum Nodes on a cable segment.

On any 1 cable segment, the maximum allowed number of nodes is 30.

Minimum Distance between Nodes

Minimum distance between nodes is 0.6 m or 2 feet.

Velocity of propagation

The speed of the signal through the 10Base2 cable is 0.65c. ("c" is equal to the speed of light - 300,000,000 m/sec). The minimum velocity of propagation for 10Base2 specification cable is equal to 0.65 x 300,000,000 m/sec. This is determined by cable capacitance.

Maximum coaxial cable segment length 185 m.

The maximum segment length is 185 m (600 ft.) or a maximum 0.949 uSec propagation delay. Propagation delay not distance is what actually determines the maximum length of the segment. Propagation delay (units are seconds) is calculated by:

What is the propagation delay for a 185 m length of 10Base2 cable?

Maximum Number of Segments

Maximum of 5 segments (with 4 repeaters) can be along the path between any 2 network nodes: 3 may be coax segments having a maximum delay of 0.949 uSec and 2 may be link segments having a maximum delay of 0.949 uSec.

With no link segments used 3 populated coax segments can exist on a path.

Maximum Transfer Rate

The Maximum Data Transfer Rate for IEEE 802.3a is 10 Mbps (10,000,000 bits per second of data). In actual fact, data transfer is dependant on how many users are fighting for the bus and how fast the user's data can get on the bus.

Physical Bus/Logical Bus

IEEE 802.3a is a Physical Bus - the cable is physically laid out as 1 long cable with the network nodes attached to it.

It is also treated as a Logical Bus - electronically and logically it appears as 1 long cable with the network nodes attached to it.

36e. IEEE 802.3i 10BaseT

Twisted Pair Cable

10BaseT uses unshielded twisted pair (UTP) cable. The cable is flexible and easy to work with. The cable has a characteristic impedance of 100 ohms. There are 2 pairs of twisted wires used with 10BaseT. Separate Rx (receive) and Tx (transmit) pairs are used. The lines are balanced lines to minimize noise and there are a Rx+ & Rx- pair and a Tx+ & Tx- pair.

The nodes are connected to a MPR (multiport repeater) also called a Concentrator or Hub. The cables are wired as straight-through cables meaning the Node's Rx & Tx lines connect directly to the Hub's Rx & Tx lines respectively.

Two nodes can be directly connected together bypassing the Hub by using a Cross-over (X-over) cable. In a X-over cable, the Tx and Rx lines are crossed so that one node's Tx lines go to the other nodes Rx lines and vice versa.

Cable Termination and Connector

The standard termination is 100 ohms. The end connector is an "RJ45" quick disconnect connector. The cable is internally terminated at the NIC and Hub.

Grounding

To minimize noise on the segment, the cable is a balanced line with Rx- & Rx+ and Tx- & Tx+. There is no shielding and any noise that appears on the Rx+ wire will appear on the Rx- wire. When the 2 signals are combined, the noise cancels due to Rx- & Rx+ being 180 degrees out of phase.

Maximum Nodes

For 10BaseT, the maximum allowed number of nodes is 128 on one segment.

Maximum Distance between Nodes & Hub

Maximum distance between nodes & Hub is 100 m.

Velocity of propagation

The speed of the signal through the cable is 0.59c. ("c" is equal to the speed of light - 300,000,000 m/sec). The minimum velocity of propagation for 10Base5 specification cable is equal to 0.59 x 300,000,000 m/sec. This is determined by cable capacitance.

Maximum cable segment length 100 m

The maximum segment length is 100 m or a maximum 0.565 uSec propagation delay. Propagation delay not distance is what actually determines the maximum length of the segment. Propagation delay (units are seconds) is calculated by:

What is the propagation delay for a 100 m length of 10BaseT cable?

Maximum Number of Segments

Maximum of 5 segments (with 4 repeaters) can be along the path between any 2 network nodes: 3 may be coax segments having a maximum delay of 0.565 uSec and 2 may be link segments having a maximum delay of 0.565 uSec. The 5-4-3 rule will be discussed under Repeaters and its special implications for IEEE 802.3i.

Maximum Transfer Rate

The Maximum Data Transfer Rate for IEEE 802.3i is 10 Mbps (10,000,000 bits per second of data). In actual fact, data transfer is dependant on how many users are fighting for the bus and how fast the user's data can get on the bus.

Physical Star/Logical Bus

IEEE 802.3a is a Physical Star - the cable is physically laid out as star pattern with all twisted pair cables (AUIs) coming from the nodes to a central wiring closet containing the Hub (Multi-Port Repeater / Concentrator)

It is treated as a Logical Bus - electronically and logically it appears as 1 long cable with the network nodes attached to it. A node can be a client, server, workstation or other hub.

36f. MAC - Medium Access ControlThe IEEE 802.3 Medium Access Control layer is physically located in the firmware (ROM) of the Network Interface Card. It is the link between the Data Link Layer and

the Physical Layer of the OSI model and logically resides in the lower portion of the Data Link Layer. There is only 1 MAC layer for all IEEE 802.3 versions: 802.3, 802.3a, 802.3b, 802.3i, etc..

The IEEE 802.3 Medium Access Control uses CSMA/CD (Carrier Sense Multiple Access/Collision Detect) to determine Bus Arbitration. The MAC layer is concerned with the order of the bits and converting the Datagram from the Network Layer into Packets/Frames.

Preamble

The Preamble is used to synchronize the receiving station's clock. It consists of 7 bytes of 10101010.

Start Frame Delimiter (SFD)

The Start Frame Delimiter indicates the start of the frame. It consists of 1 byte of 10101011. It is an identical bit pattern to the preamble except for the last bit.

Start Frame Delimiter (SFD)

The Start Frame Delimiter indicates the start of the frame. It consists of 1 byte of 10101011. It is an identical bit pattern to the preamble except for the last bit.The Destination Address (DA)

Indicates the destination (receiving station) of the frame. It can be 2 or 6 octets long (16 or 48 bits), usually it is 6 octets (the 2 octet version is used for compatibility with the original Ethernet frame from XNS and is considered obsolete).

The DA field consists of

I/G stands for Individual/Group. It indicates whether the destination is for an individual or for a multicast broadcast. It is one bit long:

0 = Individual 1 = Group

A multicast broadcast can be for everyone or for a group. For a multicast broadcast to all stations, the Destination Address = FFFFFFFFFFFFh (h - hexadecimal notation). To multicast to a specific group, unique addresses must be assigned to each station by the Network Administrator.

U/L stands for Universal/Local. It allows for unique addresses. It is used to indicate whether a local naming convention is used - administered by the Network Administrator (not recommended - incredible amount of work) or the burnt-in ROM address is used (recommended).

The 46 Bit Address Field consists of 46 bits indicating the destination NIC cards address burnt into the firmware (ROM) of the card or the unique name assigned to the card during the card's initialization by the Network Administrator.

Source Address (SA)

The Source Address indicates the source or transmitting station of the frame. It is identical in format to the Destination Address but always has the I/G bit = 0 (Individual/Group Bit = Individual)

Length (L)

The Length field indicates the Length of the Information Field. It allows for variable length frames. The minimum Information Field size is 46 octets and the maximum size is 1500 octets. When the Information Field size is less than 46 octets, the Pad field is used. Due to the 802.3 MAC Frame having a Length field, there is no End

Delimiter in the MAC Frame. The Length of the field is known and the receiving station counts the number of octets.

Information Field (Data)

The Information Field contains the Data from the next upper layer : Logical Link Control Layer. It is commonly referred to as the LLC Data. The minimum Information Field size is 46 octets and the maximum size is 1500 octets.

Pad

The Pad is used to add octets to bring the Information Field up to the minimum size of 46 octets if the Info Field is less than the minimum.

Frame Check Sequence (FCS)

The Frame Check Sequence is used for error-checking at the bit level. It is based on 32 bit CRC (Cyclic Redundancy Checking) and consists of 4 octets (4 x 8 = 32 bits). The FCS is calculated according to the contents of the DA, SA, L, Data and Pad fields.

36g. Total Length of a MAC Frame

Min Size (octets)

Max Size(octets)

Preamble 7 7Start Frame Delimiter 1 1Destination Address 6 6Source Address 6 6Length 2 2Information Field 46 1500Frame Check Sequence 4 4TOTAL: 72 1526 Octets

36i. Packet Sniffing

A packet sniffer captures packets from the Ethernet bus. The network interface card (NIC) acts in a mode called promiscious mode. Promiscious mode means that the NIC can look at all traffic on the wire and not just to traffic addressed to itself. Normally, the NIC ignores all traffic except for packets addressed to itself, multicasts and broadcast packets.

The following captured packet is displayed in raw format. Raw format is hexadecimal numbers in rows of 16 digits.

FF FF FF FF FF FF 00 20 AF 10 9A C0 00 25 E0 E0

03 FF FF 00 22 00 11 00 00 00 00 FF FF FF FF FF

FF 04 52 00 00 00 00 00 20 AF 10 9A C0 40 0B 00

01 00 04 00 00 00 00 00 00 00 00 00

Raw Captured Packet

Raw captured packets do not display the Preamble, Start Frame Delimiter and the Frame Check Sequence fields. These fields are used to inform the receiving station of a new frame and for error checking.

The breakdown of the packet is according to the Ethernet MAC frame and as follows:

1st 6 bytes: FF-FF-FF-FF-FF-FF Destination MAC address2nd 6 bytes: 00-20-AF-10-9A-C0 Source MAC addressNext 2 bytes: 0025 Length/Type field - IEEE 802.3 frameNext 37 bytes Data

Last 9 bytes all 00s Pad

The length of the data in the Info field is 0025h or 37 bytes long. The minimum Info field size is 46 bytes so the data is padded with 9 bytes of 00h.

The Length/Type field value is less than 05DCh (1500 in decimal) which indicates that it is an Ethernet_802.2 frame (IEEE 802.3) with a Logical Link Control layer (covered later) between the MAC layer and the Network layer.

If the value was 0800h, it would indicate an Ethernet_II frame used for TCP/IP.

If it were 8137, it would indicate an Ethernet_802.3 (raw) frame used by pre 3.12 Netware.

The complete listing of the Length/Type field assignments is covered in Appendix C Ethernet Type Field.

Looking at the MAC block diagram, the data from the Info field is shown broken down (up to be more exact) into the higher levels: Logical Link Control layer, Network layer and the Transport layer. Note: A thorough knowledge of each of the layers and quite a few handy reference books are required in order to determine exactly what is happening. The remaining layers will be examined as an example only.

NOTE: Modern packet sniffer will break down the raw packet's fields for you.

LLC Layer

The first 3 bytes of the data in the Ethernet frame Info field is the header of the Logical Link Control layer (LLC IEEE 802.2).

1st byte: E0 Destination Service Access Port (DSAP)

2nd byte: E0 Source Service Access Port (SSAP)

3rd byte: 03 Control code

E0h indicates that it is a Novell Netware stack talking (source) to a Novell Netware stack (destination). The 03h is the LLC layer's handshaking. The size of the LLC's Data field is 34 bytes. The LLC layer is covered extensively in the following chapter.

Network Layer

The data of the LLC layer becomes the header and data of the layer above it which is the Network layer. In this case, it is an IPX PDU (Protocol Data Unit) which is indicated by the first 2 bytes being FFFFh - the IPX checksum.

(Hex)

1st 2 bytes: FFFF IPX Checksum (always FFFFh, FCS does error checking)

Next 2 bytes: 0022 IPX PDU length allowable range 001Eh (30) to 0240h (576)

Next byte: 00 Transport control field - hop count, allowed 00 to 0Fh (15)

Next byte: 11 Packet Type 11h (17) is Netware Core Protocol (NCP)

Next 4 bytes: 00000000 Destination network address, all 0s indicate local network

Segment number in server autoexec.ncf file

Next 6 bytes: FFFFFFFFFFFF Destination host address (same as dest MAC address)

Next 2 bytes: 0452 Destination socket , Service Advertising Protocol

Next 4 bytes: 00000000 Source network address (all 0s indicate local network)

Next 6 bytes: 0020AF109AC0 Source host address (same as soruce MAC address)

Next 2 bytes: 400B Source socket (arbitrarily assigned starting at 4000h)

Last 4 bytes: Data

The following tables describe the field values for the IPX PDU's packet type and Socket numbers:

Packet Type Field Value Purpose

NLSP 00h Netware Link Services Protocol

RIP 01h Routing Information Protocol

SAP 04h Service Advertising Protocol

SPX 05h Sequenced Packet Exchange

NCP 11h Netware Core Protocol

NetBIOS 14h NetBIOS and other propagated packets

IPX Packet Type FieldNetware Socket Numbers and Processes

Socket Number Process

451h Netware Core Protocol (NCP)

452h Service Advertising Protocol (SAP)

453h Routing Information Protocol (RIP)

455h Novell NetBIOS

456h Diagnostics

9001 Netware Link Services Protocol (NLSP)

9004 IPXWAN Protocol

Transport Layer

The Network layer's Data field becomes the Transport layer's PDU. In this case it is only 4 bytes long.

1st 2 bytes: 0001 Packet type (Standard Server Request)

Next 2 bytes: 0004 Service type (file server)

The following tables describe the values of the Service Advertising Protocol's Packet Type and Service Type fields:

Field Value (hex) Packet Type

01 Standard Server Request

02 Standard Server Reply

03 Nearest Server Request

04 Nearest Server Reply

SAP Packet Types

Field Value (hex) Service Type

0000 Unknown

0003 Print Queue

0004 File Server

0005 Job Server

0007 Print Server

0009 Archive Server

0024 Remote Bridge Server

0047 Advertising Print Server

8000 All values are reserved up to 8000

FFFF Wildcard

Example Packet Sniffing Summary

This packet is commonly called a Standard Server Request that is broadcast (Destination FF-FF-FF-FF-FF-FF) on the local network (00-00-00-00) from a Novell Netware client. The client is looking for a file server to login in to. The server would respond with a Server Advertising Protocol PDU listing its services.

37. IEEE 802.2 LLC - Logical Link Control LayerThe Logical Link Control Layer resides in the upper portion of the Data Link Layer. The LLC layer performs these functions:

a. Managing the data-link communicationb. Link Addressing

c. Defining Service Access Points (SAPs)

d. Sequencing

The LLC provides a way for the upper layers to deal with any type of MAC layer (ex. Ethernet - IEEE 802.3 CSMA/CD or Token Ring IEEE 802.5 Token Passing).

The Data field of the MAC layer Frame transmits the LLC Protocol Data Unit.

LLC PDU Format

37a. Service Access Ports (SAPs)

SAPs are Service Access Ports. A SAP is a port (logical link) to the Network layer protocol. If we were operating a multiprotocol LAN, each Network Layer protocol would have its own SAP. This is the method that the LLC uses to identify which protocol is talking to which. For example, Unix's TCP/IP, Novell's SPX/IPX and IBM's Netbios would all have different SAPs to identify which was which.

Address Assignment

00 Null LSAP

02 Individual LLC Sublayer Management Function

03 Group LLC Sublayer Management Function

04 IBM SNA Path Control (individual)

05 IBM SNA Path Control (group)

06 ARPANET Internet Protocol (IP)

08 SNA

0C SNA

0E PROWAY (IEC955) Network Management & Initialization

18 Texas Instruments

42 IEEE 802.1 Bridge Spannning Tree Protocol

4E EIA RS-511 Manufacturing Message Service

7E ISO 8208 (X.25 over IEEE 802.2 Type 2 LLC)

80 Xerox Network Systems (XNS)

86 Nestar

8E PROWAY (IEC 955) Active Station List Maintenance

98 ARPANET Address Resolution Protocol (ARP)

BC Banyan VINES

AA SubNetwork Access Protocl (SNAP)

E0 Novell NetWare

F0 IBM NetBIOS

F4 IBM LAN Management (individual)

F5 IBM LAN Management (group)

F8 IBM Remote Program Load (RPL)

FA Ungermann-Bass

FE ISO Network Layer Protocol

FF Global LSAP

DSAP stands for Destination Service Access Port and is the receiving station's logical link to the Network Layer protocol. SSAP stands for Source Service Access Port and is the transmitting station's logical link to the Network Layer Protocol.

SAPs ensure that the same Network Layer protocol at the Source talks to the same Network Layer protocol at the Destination. TCP/IP talks to TCP/IP, Netbios talks to Netbios and IPX/SPX talks to IPX/SPX.

37b. Types of LLC Operation

LLC defines 2 types of operation for data communication:

Type 1: Connectionless Type 2: Connection Oriented

Type 1: Connectionless

Connectionless service for data communications is very similar to sending mail with the postal system (hand delivered mail). The data is sent and we hope it arrives at its destination. There is no feedback from the destination to indicate whether it arrived or not.

Type 1: Connectionless Service

Type 2: Connection Oriented

Connection Oriented service for data communications is very similar to having a phone conversation. First a connection is made and established by dialing the number, waiting for it to ring, someone picking up the line and saying hello. This establishes the connection. During the conversation, confirmation that the other person is still there (hasn't fallen asleep or died) and listening is given by hearing things like: yeah, oh really, uh huh, etc.. This is the acknowledgement of receipt of data. If the destination party did not hear something correctly, they ask to have it repeated which is called automatic repeat request (ARQ).

Connection Oriented service

NOTE: These models for connectionless and connection-oriented can be used for any protocol.

Type 2: Connection Oriented operation for the LLC layer provides 4 services:1. Connection establishment

2. Confirmation and acknowledgement that data has been received.

3. Error recovery by requesting received bad data to be resent.

4. Sliding Windows (Modulus: 128)

Sliding Windows are a method of increasing the rate of data transfer. Type 2 Connection Oriented operation calls for every Protocol Data Unit (LLC frame) sent to be acknowledged. If we waited for every PDU to be acknowledged before we sent the next PDU, we would have a very slow data transfer rate.

For example: If we were contacting Microsoft in Sunnyvale California, it might take 2 seconds for our LLC PDU to reach Microsoft and another 2 seconds for the acknowledgement to return. This would mean that we are only sending 1 PDU every 4 seconds. If our PDU was IEEE 802.3 MAC's limit of 1500 octets (8x1500 = 12 Kbits), we would actually be transferring at 3 Kbps (12 kbits/4 seconds). This would be regardless of our actual transfer rate! Waiting for an acknowledgement is controlling the data transfer rate!

To overcome this problem, a Sliding Window system of data transmission is used. Each PDU is sequentially numbered (0 - 127). Rather than wait for an acknowledgement, the next PDU is numbered and sent out. The receive station LLC layer acknowledges with the received PDU's numbers back to the transmit station. The LLC will allow up to 128 PDUs to be sent and not acknowledged before it sounds an error alarm.

The received station LLC layer keeps track of the PDUs it is receiving and if one should be lost during transit, it requests the Source to restart transmitting at that PDU number. All PDUs since the lost PDU are discarded.

It is called a Sliding Window because the number of unacknowledged PDUs is determined by the time it takes to get to the destination and for the destination to acknowledge the receipt of the PDU. This time is dependant on the transfer rate and the physical distance the PDU must travel. It is set automatically and we do not have to worry about it.

37c. Classes of LLC

There are 2 Classes of Logical Link Control defined:

- Class I : Type 1 operation only (connectionless)

- Class II: Both Type 1 (connectionless) and Type 2 (connection-oriented) operation allowed.

37d. LLC PDU Control Field Formats

There are 3 LLC PDU Control field formats:

a) Un-numbered (U-Format PDU) 

b) Information Transfer (I-Format PDU) c) Supervisory (S-Format PDU)

Un-numbered (U-Format PDU)

The last 2 bits are set to 1, to indicate U-Format Control Field.

M - Modifier bits, they are set depending on the mode of operation: Command, Response or Data

P/F - Poll/Final bit, this bit is used by the Source to solicit a response from the Destination. It is used by the Destination to respond to a solicit from the Source.

The Un-numbered LLC Control field is used mainly in Type 1 (connectionless) operation. The PDUs are not numbered, they are sent out and hopefully arrive at their destination. U-Format PDUs can be commands, responses and data. There are only 8 bits in a U-Format LLC PDU. In the U-Format (Unnumbered), there are 8 commands & responses:

UI - Unnumbered information (here's some data - hope it arrives)

DISC - Disconnect (we're done, shut her down)

SABME  - Set Asynchronous Balanced Mode Extended (start now)

XID - Exchange IDs (Here's who I am, who are you?)

TEST - Test the link (Here's a test, send me back a test)

UA - Unnumbered Acknowledgement (Yes, I'm still here)

DM - Disconnect Mode (I'm disconnecting)

FRMR - Frame Reject (Bad frame - reject)

Information Transfer (I-Format PDU)

It is used for transferring information or data between Source and Destination in a Type 2 (connection oriented) operation. It is the only LLC PDU allowed to transfer information in Type 2 operation.

I-Format Control Field Format

The last bit is set to 0, to indicate that it is an I-Format Control Field.

P/F - Poll/Final bit, this bit is used by the Source to solicit a response from the Destination. It is used by the Destination to respond to a solicit from the Source.

N(R) - PDU number received. Used with the Sliding Window and for acknowledging PDUs.

N(S) - PDU number sent. Used with the Sliding Window and for acknowledging PDUs.

The N(R) bits are commonly called "Piggyback Acknowledgment" because the response is acknowledged along with the transfer of data. The acknowledgement is piggybacked onto a data transfer.

In the I-Format (Information), there are no commands & responses but typically indicated by:

I - Information (data transfer)Supervisory (S-Format PDU)

Supervisory (S-Format) LLC Control fields are used for Data Link supervisory control functions (handshaking). The S-Format Control fields are used for acknowledging I-Format PDUs, requesting retransmission, requesting a temporary suspension of transmission (buffers full - wait).

S-Format LLC PDU Control Field

The last 2 bits are set to 0 1, to indicate that it is a S-Format Control Field

S - Supervisory function bits. Determines the purpose of the control field

The four 0s in a row are reserved bits and are always set to 0.

P/F - Poll/Final bit, this bit is used by the Source to solicit a response from the Destination. It is used by the Destination to respond to a solicit from the Source.

N(R) - PDU number received. Used with the Sliding Window and for acknowledging PDUs.

In the S-Format (Supervisory), there are 3 commands & responses:

RR - Receive Ready (awake & ready to receive)

RNR - Receive Not Ready (got problems, hold off for awhile)

REJ - Reject (received a bad PDU, send the PDU with this number again)

38. Network Interface CardsThere are 3 configuration types of Network Interface Cards (NIC):

1. jumper configurable2. software configurable

3. Plug n Play (PnP)

Jumper configurable cards have physical jumpers that you use to select the IRQ, I/O address, upper memory block and transceiver type (10BaseT, 10Base2 or 10Base5). Older cards will also allow selecting DMA channel - this was used with XT and 286 PCs.

Software configurable NICs have a proprietary software program that sets the NIC's "internal jumpers". They are usually menu driven and have an auto configuration mode, where the program will attempt to determine the most suitable configuration. These programs are not foolproof, you still require a thorough knowledge of the PC's architecture.

Plug n Play NICs will attempt to auto-configure themselves during the bootup sequence immediately after installation. They also come with a proprietary software program in case that anything goes wrong and you have to manually configure them.

A combination (combo) NIC has the option of connecting to the network using either Twisted Pair (10BaseT), Coax (10Base2) or AUI (Attachment Unit Interface for 10Base5). The NIC can only connect to one medium type at a time and the configuration software allows you to select which medium interface to connect to. Newer NICs will autodetect the cabling type used.

38a. IRQs, DMAs and Base Addresses

When a NIC is configured, you are setting the parameters which tell the computer network software where to find the adapter (base address) and who is "tapping the CPU on the shoulder" (IRQ). The base address is the pointer to the rest of the world that says "Here I am at base address xxx!". The IRQ is the "tap on the shoulder" to the CPU that says "Hey, it's IRQx, I've got something important to say!". The Upper Memory Block is the NIC's BIOS or actual program in the NIC's ROM. It is set to a free area of memory in the PC's upper memory - to avoid conflicts with other devices (video cards, internal modems, SCSI drivers, etc..).

IRQ - Interrupt Requests

IRQ stands for Interrupt ReQuest. It is a "tap on the shoulder" to the CPU by a peripheral card plugged in an ISA slot to tell the CPU that the peripheral has something to say (also used by EISA and MCA slots). Common peripherals are modems, NICs (network interface cards), sound cards, SCSI adapters, hard-drive controllers, floppy drive controllers, COM ports and printer ports.

An IRQ is a hardware interrupt, this means that there is a physical line run to each of the ISA slots on the motherboard. There are 2 types of ISA slots: 8 bit and 16 bit. The 16 bit consists of the 8 bit slot plus a 16 bit extension slot. There are 8 IRQ (IRQ0-7) lines that run to the 8 bit ISA slot. There are 8 more (IRQ8-15) that run to the 16 bit ISA extension slot. For a total of 16 IRQs in a typical ISA bus PC.

IRQ0 has the highest priority and IRQ7 the lowest priority. IRQ8-15 have "special" priority as will be explained. When IBM introduced the AT computer, they added IRQ8-15. In order to make AT (286) PCs backward compatible with 8 bit XT (8088) PCs and to "up" the priority of the new IRQ lines, they cascaded two interrupt controllers. This results in IRQ8-15 having the same priority as IRQ2. Priority means if two IRQs are active at the same time, the one with the higher priority is serviced first.

IMPORTANT: An IRQ can be assigned to only ONE active device at a time. If 2 devices share the same IRQ, this is called a CONFLICT. This means that when the IRQ line becomes active, the CPU does not know which device needs to "talk".

For example, if a modem used IRQ5 and a NIC used IRQ5. When the modem had some information that needed to be passed on to the CPU, it would set IRQ5 active. The CPU would not know whether to talk to the NIC or modem. The computer may hang, or nothing would happen.

*** IRQ conflicts are the NUMBER 1 source of PC problems! ***

Here is a table that is used as a rule of thumb (guideline) in selecting IRQs for PCs. The IRQs are listed in order of priority. (Note that not all IRQ lines go to the card slots)

IRQ Function Physical Line ISA Bus

IRQ0 System Timer No

IRQ1 Keyboard Controller No -

IRQ2 Cascaded to IRQ8-15 No -

IRQ8 Real-time clock No -

IRQ9 *-Available (IRQ2) Yes 8/16 bit

IRQ10 NIC Yes 16 bit

IRQ11 SCSI adapter Yes 16 bit

IRQ12 Motherboard mouse/available Yes 16 bit

IRQ13 Math coprocessor No -

IRQ14 Primary IDE controller Yes 16 bit

IRQ15 Secondary IDE controller Yes 16 bit

IRQ3 Com2/Com4 Yes 8 bit

IRQ4 Com1/Com3 Yes 8 bit

IRQ5 Sound card/LPT2 Yes 8 bit

IRQ6 Floppy drive controller Yes 8 bit

IRQ7 Parallel port LPT1 Yes 8 bit

*- IRQ9 appears as if it is IRQ2. Normally not used because it can cause interesting problems to appear. Is it really IRQ9 or is it the IRQ2 cascaded to IRQ9? Which do you set it to? What if you are using an 8 bit ISA modem in a 16 bit ISA slot? See what I mean...

The preceding table is a rule of thumb or guideline to selecting IRQs for your peripherals. For example if the PC does not use a SCSI adapter than IRQ11 is available for use for another NIC card or another device. Most autodetecting software or operating systems expect to see the IRQs assigned as above.

Standard COM Port Assignment

Note that COM1 (DB9 on the back of the PC) and COM3 share IRQ4. This is allowed as long as only one device is active at a time. This means that if you are running a mouse on COM1 then you cannot use COM3 for an internal modem. You will run into a conflict.

Some communication packages will allow you to do this but most will choke or cause flaky operation. A common sympton is if you move the mouse, you see garbage on your terminal program.COM2 (DB25 on the back of the PC) and COM4 have a similar problem except that most people don’t use COM2. It is usually safe to configure an internal modem to COM4. If COM2 is used, it is typically used for an external modem or a plotter. Usually, both are not active at the same time.

Port IRQ Function

COM1 4 Mouse

COM2 3 Not used or plotter or external modem

COM3 4 Not used (conflicts with mouse)

COM4 3 Not used or internal modem

DMA -Direct Memory Access

DMA stands for Direct Memory Access. This is a method that allows channels to be openned by the peripheral to read/write directly to memory without going through the CPU. This off-loads some of the work from the CPU to allow it to do more important tasks.

There are 8 DMA channels available in the PC: DMA0-7. They are divided into 8 bit channels and 16 bit channels based on the 8 bit ISA slot and 16 bit ISA slot. Here is a table that is used as a rule of thumb for selecting DMA channels:

DMA Function Physical Line ISA Bus Channel Width

DMA0 Available Yes 16 bit 8 bits

DMA1 Sound card Yes 8 bit 8 bits

DMA2 Floppy Disk controller Yes 8 bit 8 bits

DMA3 ECP Parallel Port Yes 8 bit 8 bits

DMA4 * - Not used No - 16 bit

DMA5 Sound card Yes 16 bit 16 bit

DMA6 SCSI Yes 16 bit 16 bit

DMA7 Available Yes 16 bit 16 bit

* - DMA4 is cascaded to the first 8 bit DMA controller and is not available.Note: DMA0 is on the 16 bit ISA bus but is only 8 bits wide.

*** DMA conflicts are the NUMBER 2 source of PC problems! ***

Like IRQs, you are only allowed to assign one DMA channel to an active device at a time. Otherwise you will have a conflict appear and things will not work properly.

You may have one DMA channel assigned to two devices ONLY if one device is active at a time.

Base Addresses

Base addresses are also called I/O ports, I/O addresses, I/O port addresses and base ports. They are memory locations that provide an interface between the operating system and the I/O device (peripheral). The peripheral communicates with the operating system through the base address. Each peripheral must have a UNIQUE base address. Standard Base Address assignments (h - hexadecimal):

Base Address Function

060h + 064h Keyboard controller

170h + 376h Secondary IDE Hard-drive controller

1F0h + 3F6h Primary IDE Hard-drive controller

220h Sound Card

2A0h Token Ring NIC

300h Ethernet NIC

330h SCSI adapter

3F2h Floppy Drive Controller

3F8h COM1

2F8h COM2

3E8h COM3

2E8h COM4

378h LPT1

278h LPT2

*** Base Address conflicts are the NUMBER 3 source of PC problems! ***

Unfortunately, the above table is only a small part of the Base Addresses used. The base addresses used will depend on what has been installed on the PC.

38b. Legacy NICs

Before installing a legacy (polite way of saying old) NIC, a PC diagnostic program (Checkit or MSD) should be run to determine available: IRQs, Base Addresses and UMBs. After determining which IRQs, Base Addresses and UMBs are available, you would configure the NIC hopefully to the rule of thumb tables listed previously. In the case of the Upper Memory Block, you would also allocate that memory block using EMM386.EXE in config.sys (x800 block size).

Ex: device=c:\dos\emm386.exe x=C000-C800

This would ensure that EMM386.EXE does not allow any other program, Windows or TSR from using the same memory block thus avoiding memory conflicts. This is used to be a typical job interviewer's question: "What do you do to config.sys when installing a legacy network card?".

38c. NIC Diagnostic Tools

NICs come with software diagnostic tools that allow you to check the operation of the NIC. They are usually called Internal Diagnostics, Loopback Test and Echo Server Test. The Internal Diagnostics checks the internal hardware on the NIC card. It usually checks about a dozen or more different aspects of the network card up to the transmit/receive circuitry.

Internal Diagnostics

Loopback Test checks to see if the NIC can transmit and receive data properly. This test is usually applicable to 10Base2 (coax) only, as a BNC TEE with 2 terminations is required. There is no 10BaseT loopback test because you can't terminate at the NIC.

Loopback Test

Note: The first two diagnostic routines are performed not connected to the physical network. This prevents faulty NICs from disruptting normal network traffic. The last

diagnostic routine is the Echo Server or Network test. Two NICs are required. A known working NIC acts as an Echo Server and the NIC under test is the Echo client. The echo client sends a packet to the echo server who echoes the packet back. This is tested on the network and can be used for any cabling type not just 10Base2 as per the example.

Echo Server Test

38d. Network Interface Card Drivers

Network Interface Card Drivers are the software interface between the Network Card Hardware/Firmware and the Network Operating System Data Link layer. The Network Card device driver is a device driver loaded in config.sys. The Network Card consists of Firmware and Hardware.

The Firmware is the program stored on the network card's ROM (BIOS) and configuration information stored in E2ROM. The configuration information would be the IRQ, Base Memory Address, Transceiver Type, etc.. for the Network Card. The Hardware would be the physical components: ICs, connectors, etc..

There are basically 3 types of Network Card Drivers:

NDIS ODI

Packet drivers

NDIS stands for Network Driver Interface Specification. NDIS drivers are used by Microsoft based Network Operating Systems such as Microsoft LAN Manager, Windows NT, Windows for WorkGroups and IBM's OS/2.

ODI stands for Open Datalink Interface. ODI drivers are used by Novell's Network Operating System and Apple.

Packet drivers use software interrupts to interface to the network card. Many non-commercial programs (shareware and freeware) use Crnywr packet driver interfaces.

The 3 Network Driver Types are not compatible with each other but most Network Operating Systems (Novell, WFWG, etc..) can use either NDIS or ODI. The NOS (Network Operating System) determines which type of Network Driver can be used. Regardless of the Network Driver type used, all have a network device driver loaded into memory during boot up and a network protocol bound to the network card.

The purpose of the Network Drivers is to decouple the network adapter's device driver from the higher layer protocols. The higher layer protocols can be IPX/SPX for Novell, Netbios for Microsoft, TCP/IP for Unix etc..

Traditional Network Card Device Driver Problems (pre-1990)

Traditionally (in the olden days - 1990), the Network Card Device Driver and NOS' Data Link layer were generated as 1 software program specific to the computer it was generated on.

As an example, with Novell 3.11 and earlier, a special program was run, called WSGen (workstation generator), which would generate a Workstation Shell. The Workstation Shell would be a software program running as a TSR which would be a combination of the Network Card Device Driver and Novell's IPX protocol. The Workstation Shell was specific to the computer that it was generated on and could not be used on another computer. This meant that every PC in a network would have its Workstation Shell recompiled with every new version of Novell! In a small network this would not be a problem, but in large networks (100+ PCs), this becomes a logistic nightmare!

Another problem emerged, the Workstation Shells directly controlled the network card and were specific to only one NOS. This meant that only one NOS protocol could be run, in Novell's case IPX. Interconnecting Networks became a major problem.

Still another problem arose in trying to run more than 1 network card in a computer (This is done typically in bridges, routers and servers). The Workstation Shells did not have the provision to allow the NOS Protocol to "bind" to more than one Network Card easily.

The NIDS and ODI Network Card Driver specifications were implemented to address the following specific areas:

Provide a standard separate interface between the Network Card Device Driver and Data Link Layer.

Allow more than one NOS Protocol to access the Network Card Device Driver.

Allow the NOS to "bind" to more than one Network Card Device Driver.

NDIS Drivers The NDIS (Network Driver Interface Specification) standard was developed

jointly by Microsoft and 3Com for implementation in Microsoft's NOS and IBM OS/2.

The Microsoft implementation of NDIS modifies the config.sys file, autoexec.bat file and makes two important initialization files: SYSTEM.INI and PROTOCOL.INI.

Microsoft loads the IFSHLP.SYS file as a device driver in the CONFIG.SYS file. The IFSHLP.SYS is the installable file system helper file and contains the network redirector for the NDIS interface. The LASTDRIVE command in the config.sys file tells the network operating system the last available drive that can be used for mapping network drives.

The SYSTEM.INI file contains information similar to the following: [network] sizworkbuf=1498 filesharing=no printsharing=no autologon=yes computername=E237-12 lanroot=C:\NET

username=EBLANCHARD workgroup=WORKGROUP reconnect=yes dospophotkey=N lmlogon=1 logondomain=T217PROJECT preferredredir=full autostart=full maxconnections=8 [network drivers] netcard=elnk3.dos transport=ndishlp.sys,*netbeui devdir=C:\NET LoadRMDrivers=yes [Password Lists] *Shares=C:\NET\Shares.PWL EBLANCHARD=C:\NET\EBLANCHA.PWL The PROTOCOL.INI file contains protocol specific information and the

virtual network card interface. A typical netbeui NDIS protocol.ini looks like: [network.setup] version=0x3110 netcard=ms$elnk3,1,MS$ELNK3,1 transport=ms$ndishlp,MS$NDISHLP transport=ms$netbeui,MS$NETBEUI lana0=ms$elnk3,1,ms$netbeui lana1=ms$elnk3,1,ms$ndishlp

[protman] DriverName=PROTMAN$ PRIORITY=MS$NDISHLP [MS$ELNK3] DriverName=ELNK3$ IOADDRESS=0x300 [MS$NDISHLP] DriverName=ndishlp$ BINDINGS=MS$ELNK3 [MS$NETBEUI] DriverName=netbeui$ SESSIONS=10 NCBS=12 BINDINGS=MS$ELNK3 LANABASE=0 ODI Drivers The Open Datalink Interface (ODI) is a software standard developed by Novell

and Apple Corporation to provide a layered approach to comply with the ISO Open System Interconnect (OSI) model for the Physical, Datalink and Network layers.

The Open Datalink Interface was developed to overcome several limitations on the previous network interface card driver software. Previous to the ODI standard, each workstation was required to "compile" its own workstation's IPX.COM shell using Novell's "WSGEN" program (workstation generation program). This resulted in a single program, that contained the network card driver, Datalink interface and Network layer protocol (IPX/SPX), commonly called the "workstation shell".

This approach limited the workstation to 1 network card and only 1 Network layer protocol. Multiple network cards and Network layer protocols were not allowed under "WSGEN".

The ODI standard broke the "workstation shell" into manageable parts that permits multiple network cards and protocols. For example: This means that 1 workstation/client can have an Ethernet 10BaseT card running IPX/SPX protocols (Novell) and a Farallon Localtalk card in it for running Appletalk (Macintosh).

The ODI standard compared to the OSI Model:

OSI = Open System Interconnect ODI = Open Datalink

Interface SPX = Sequenced Packet Exchange IPX = Internetwork

Packet Exhange LSL = Link Suppport Layer VLM = Virtual

Loadable Modules MLID = Multiple Link Interface Driver MSM = Media Support

Module HSM = Hardware Support Module

Novell Lite (very old - defunct) is Novell's Peer to Peer Network Operating system. Peer to Peer Networks use DOS's File Allocation Table (FAT) and Novell Lite is no exception (Novell Netware has its own high performance disk operating system). Novell Lite follows Novell's Netware structure for the Network, Datalink and Physical layers and it is an excellent example of an ODI compliant NOS (Network Operating System). At the Transport layer it uses Peer to Peer Client and Server software instead of Novell's Netware Transport layer software - SPX (VLM).

A typical Novell client is loaded from the DOS prompt or from a STARTNET.BAT file:

SET NWLANGUAGE= ENGLISH LSL.COM Link Support Layer Software 3C509.COM 3C509 Network Interface Card Driver

(MLID) ODI Compliant IPXODI IPX Network layer protocol driver VLM Loads client software

NET.CFG is the network configuration file used by the above files. It is a text file and contains the following basic section:

Link Driver 3C5X9 (NIC drivername) INT 10 (IRQ #) PORT 300 (Base memory address in hexadecimal) FRAME Ethernet_802.2 (Frame type on Netware 3.12 & newer) FRAME Ethernet_802.3 (Frame type on Netware 3.11 and older) FRAME Ethernet_II (Frame type used by UNIX) FRAME Ethernet_SNAP (Frame type used by Appletalk) NetWare DOS Requester FIRST NETWORK DRIVE = F USE DEFAULTS = OFF VLM = CONN.VLM VLM = IPXNCP.VLM VLM = TRAN.VLM VLM = SECURITY.VLM ; VLM = NDS.VLM (used for Netware 4.11 NDS services) VLM = BIND.VLM VLM = NWP.VLM VLM = FIO.VLM VLM = GENERAL.VLM

VLM = REDIR.VLM VLM = PRINT.VLM VLM = NETX.VLM

Packet Drivers

Packet drivers use software interrupts to identify the network cards to the data link layer. Packet drivers are free software drivers that were developed to address the problems of running multiple protocols over one network card. NDIS and ODI are proprietary schemes that have been developed by 3COM/Microsoft and Novell/Apple respectively to address this problem.

The Crynwr Software collection of packet drivers are available throughout the Internet and they are free to use unlike shareware and commercial products.

Advantages:

Run multiple applications across the same board: TCP/IP, NetBIOS, Netware One board fits all, no buying different boards for different applications.

No more reconfiguring and rebooting to change applications.

Connect to a Novell File Server (or servers) and still run TCP/IP or PC-NFS or with the Novell systems remaining active and available for file serving and printing.

The Packet Driver acts as a fast smart secretary, bothering clients only when packets arrive specifically for them.

Software Interrupts

Software interrupts are interrupts generated by software unlike hardware interrupts that are physical lines that run to each device. Software interrupts that are available are 0x60 to 0x66. Table xx-1 lists the software interrupts and their assignments.

The packet drivers are assigned software interrupts to the network interface card during the bootup process usually in the autoexec.bat file. For a 3c503 card the autoexec.bat file would have this line:

3c503 0x60 5 0x300

where:

3c509 calls up the packet driver 3c509.com 0x60 is the software interrupt assigned to the NIC

5 is the hardware interrupt of the NIC

0x300 is the I/O address of the NIC

Any network traffic received or transmitted from the NIC would be addressed by the software interrupt 0x60. Complete documentation is available from the Crynwr collection under the files. Important files to read are install.doc and packet.doc.

Software Interrupts Assignments

60 -- -- reserved for user interrupt

61 -- -- reserved for user interrupt

62 -- -- reserved for user interrupt

63 -- -- reserved for user interrupt

64 -- -- reserved for user interrupt

65 -- -- reserved for user interrupt

66 -- -- reserved for user interrupt

67 -- -- LIM EMS

68 01 -- APPC/PC

69 -- -- unused

6A -- -- unused

6B -- -- unused

6C -- -- DOS 3.2 Realtime Clock update

6D -- -- VGA - internal

6E -- -- unused

6F -- -- Novell NetWare

70 -- -- IRQ8 - AT/XT286/PS50+ - REAL-TIME CLOCK

71 -- -- IRQ9 - AT/XT286/PS50+ - LAN ADAPTER 1

72 -- -- IRQ10 - AT/XT286/PS50+ - RESERVED

73 -- -- IRQ11 - AT/XT286/PS50+ - RESERVED

74 -- -- IRQ12 - PS50+ - MOUSE INTERRUPT

75 -- -- IRQ13 - AT/XT286/PS50+ - 80287 ERROR

76 -- -- IRQ14 - AT/XT286/PS50+ - FIXED DISK

77 -- -- IRQ15 - AT/XT286/PS50+ - RESERVED

78 -- -- not used

79 -- -- not used

7A -- -- Novell NetWare - LOW-LEVEL API

7A -- -- AutoCAD Device Interface

7B -- -- not used

7C -- -- not used

7D -- -- not used

7E -- -- not used

7F -- -- HDILOAD.EXE - 8514/A VIDEO CONTROLLER INTERFACE

80 -- -- reserved for BASIC

39. RepeatersRepeaters are physical hardware devices that have a primary function to regenerate the electrical signal by:

Reshaping the waveform Amplifying the waveform

Retiming the signal

39a. Purpose of a Repeater

The purpose of a repeater is to extend the LAN Segment beyond its physical limits as defined by the Physical Layer's Standards (e.g. Ethernet is 500m for 10Base5). A LAN Segment is a logical path such as the logical bus used by all 802.3 Ethernet types. A LAN Segment is given an identification number called a Segment Number or Network Number to differentiate it from other segments.

Typically, repeaters are used to connect 2 physically close buildings together that are too far apart to just extend the segment. Can be used to connect floors of a building together that would surpass the maximum allowable segment length. Note: for large extensions as in the above example, 2 Repeaters are required. For shorter extensions, only 1 Repeater may be required.

39b. Repeater's OSI Operating Layer

Repeaters operate at the OSI Model Physical Layer.

39c. Repeater's Segment to Segment Characteristics

Repeaters do not "de-segment" a network. All traffic that appears on one side of the repeater appears on both sides. Repeaters handle only the electrical and physical characteristics of the signal.

Repeaters work only on the same type of Physical Layer: Ethernet to Ethernet or Token Ring to Token Ring. They can connect 10Base5 to 10BaseT because they both use the same 802.3 MAC layer.

You can run into problems connecting 1Base5 to 10BaseT with the transfer rate (1 Mbps vs. 10 Mbps). A repeater cannot connect Token Ring to Ethernet because the Physical Layer is different for each.

39d. Repeater Addressing: MAC Layer and Network Segment

The MAC Layer Address is used to identify the Network Card to the Network. The Repeater is transparent to both sides of the segment and both sides can "see" all the Mac Addresses regardless on which side they are on. This means that any network traffic on Floor 1 will appear on Floor 5 and vice versa.

Nodes A & B could be furiously exchanging files and this network traffic would also appear on Floor 1. Repeaters provide no isolation between segments, there is only one collision domain.

Because Repeaters provide no isolation between segments and the repeater is transparent to both sides of the segment, both sides of the repeater appear as 1 long

segment. The Network Number or Segment Number is the same on both sides of the Repeater.

When using repeaters, make sure that the overall propagation delay does not exceed the Physical Layer Standard being used. Repeaters will add a propagation delay to the signal that is being repeated also. Check that rules such as the 5-4-3 Rule for IEEE 802.3 are not broken or for XNS Ethernet that a maximum of only 2 Repeaters are between any 2 nodes.

You are allowed to parallel Segments using multiport repeaters. Multiport repeaters have several inputs/outputs. Notice that all floors have the same Segment Number. You are not allowed to create a loop between two segments by using two repeaters.

Fibre Optic Repeaters join 2 segments together with a fibre optic link. The Transfer rate is not changed through the fibre. The advantage is noise immunity and longer distances. Segments can be joined up to 3000m apart and still be within the propagation delay specification for the Physical Layer. Two fibre optic repeaters are required: one at each end of the fibre.

Fibre Optic Repeater

40. HubsHubs are also called Multiport Repeaters or Concentrators. They are physical hardware devices.

Some Hubs are basic hubs with minimum intelligence - no microprocessors. Intelligent Hubs can perform basic diagnostics and test the nodes to see if they are operating correctly. If they are not, the Smart Hubs or Intelligent Hubs will remove the node from the network. Some Smart Hubs can be polled and managed remotely.

40a. Purpose of Hubs

Hubs are used to provide a Physical Star Topology. The Logical Topology is dependant on the Medium Access Control Protocol. At the center of the star is the Hub with the network nodes on the tips of the star.

Star Topology

The Hub is installed in a central wiring closet with all the cables extending to the network nodes. The advantage of having a central wiring location is that it is easier to maintain and troubleshoot large networks. All of the network cables come to the central hub, it is especially easy to detect and fix cable problems. You can easily move a workstation in a star topology by changing the connection to the hub at the central wiring closet.

The disadvantages to a star topology are:

failure of the Hub can disable a major section of the network The Star Topology requires more cabling than does the ring or the bus topology

because all stations must be connected to the hub, not to the next station.

40b. Hub's OSI Operating Layer Hubs are multiport repeaters and as such obey the same rules as repeaters (See

previous section OSI Operating Layer). They operate at the OSI Model Physical Layer.

40c. Hub's Segment to Segment Characteristics To understand the Ethernet segment to segment characteristics of a hub, the

first thing to do with Ethernet Hubs is to determine how they operate. Logically, they appear as a Bus Topology and physically as a Star Topology. Looking inside an Ethernet Hub, we can see that it consists of a electronic

printed circuit board which doesn't tell us much. If we form a functional drawing, we can clearly see how the Physical and Star Topology appears:

Understanding that inside the Hub is only more repeaters, we can draw the conclusion that all connections attached to a Hub are on the same Segment and have the same Segment Number. It is considered one repeater from any port to any port even though it is indicated as a path of 2 repeaters.

The 5-4-3 Rule for Ethernet Hubs: Cascaded Hub Network Cascading Hubs means to connect the Hubs together through the RJ45 ports.

One Master Hub (Level 1) is connected to many Level 2 (Slave) Hubs who are masters to Level 3 (slave) Hubs in a hierarchical tree or clustered star. The maximum number of stations in a Cascaded Hub Network is limited to 128.

Backbone Networks In a Backbone Network, there is no Master Hub. The Level 1 Hubs are

connected through their AUI port to a Coax Backbone. For Thin Coax, up to 30 Hubs can be connected together. For Thick Coax, up to 100 Hubs can be connected to the backbone. The Backbone is considered to be a populated segment.

Level 2 Hubs are allowed to be connected to the Level 1 Hubs' 10BaseT ports. This connection between the 2 Hubs is considered an unpopulated segment or link segment. Up to 1024 stations or nodes can be attached to the Level 2 Hubs' 10BaseT ports.

All stations and segments would appear as 1 Logical segment with 1 Network Number. In the real world, you would never attach 1024 stations to 1 segment, the resulting traffic would slow the network to a crawl.

40d. Hub's Addressing Again, because a Hub is just many repeaters in the same box, any network

traffic between nodes is heard over the complete network. As far as the stations are concerned, they are connected on 1 long logical bus (wire).

40e. Half-Duplex & Full-Duplex Ethernet Hubs Normal Ethernet operation is Half-Duplex: only 1 station or node talking at a

time. The stations take turns talking on the bus (CSMA/CD -bus arbitration).

Full-Duplex Ethernet Hubs are Hubs which allow 2 way communication

between Hubs thus doubling the available bandwidth from 10 Mbps to 20 Mbps. Full duplex Hubs are proprietary products and normally only work within their own manufacturer's line.

If A wanted to talk to C, a direct 10 Mbps line would be connected through the 2 switching hubs. Simultaneously, if D wanted to talk to B, another direct 10 Mbps line in the opposite direction would be connected through the 2 switching Hubs thus doubling the available bandwidth to 20 Mbps.

There are no official standards for Full-Duplex Ethernet just proprietary ones.

40f. Switching Hubs Switching hubs are hubs that will directly switch ports to each other. They are

similar to full duplex hubs except that they allow dedicated 10 Mbps channels between ports.

If A wanted to communicate with B, a dedicated 10 Mbps connection would be

established between the two. If C wanted to communicate with D, another dedicated 10 Mbps connection would be established.

41. BridgesBridges are both hardware and software devices. They can be standalone devices - separate boxes specifically designed for bridging applications, or they can be dedicated PCs with 2 NICs and bridging software. Most servers software will automatically act as a bridge when a second NIC card is installed.

41a. Bridge OSI Operating Layer

Bridges operate on the OSI Model Data Link Layer. They look at the MAC addresses for Ethernet and Token Ring to determine whether or not to forward or ignore a packet.

41b. Purpose of a Bridge

The purposes of a Bridge are: Isolates networks by MAC addresses Manages network traffic by filtering packets

Translate from one protocol to another

Isolates networks by MAC addresses

For example, you have 1 segment called Segment 100 with 50 users in several departments using this network segment. The Engineering Dept. is CAD (Computer Aided Design) oriented and the Accounting Dept. is into heavy number crunching: year end reports, month end statements etc..

On this network, any traffic between Client A, B or C and the Accounting File Server in the Accounting Dept. will be heard across the Segment 100. Likewise any traffic between the Engineering Dept.'s Clients G, H or I to the CAD File Server will be heard throughout the Network Segment. The result is that the "Other" Departments access to the Generic File Server is incredibly slow because of the unnecessary traffic occurring due to other departments: Engineering & Accounting.

Note: The designations A, B, and C are used instead of MAC addresses for brevity. The actual MAC addresses would be hexadecimal numbers such as 08-00-EF-45-DC-01.

The solution is to use a Bridge to isolate the Accounting Dept. and another bridge to isolate the Engineering Department. The Bridges will only allow packets to pass through that are not on the local segment. The bridge will first check its "routing" table to see if the packet is on the local segment, if it is, it will ignore the packet and not forward it to the remote segment. If Client A sent a packet to the Accounting File Server, Bridge #1 will check its routing table, to see if the Accounting File Server is

on the local port. If it is on the local port, Bridge #1 will not forward the packet to the other segments.

If Client A sent a packet to the Generic File Server, again Bridge #1 will check its routing table to see if the Generic File Server is on the local port. If it is not, then Bridge #1 will forward the packet to the remote port.

Note: The terms local and remote ports are abitrarily chosen to distinguish between the two network ports available on a bridge.

In this manner the network is segmented and the local department traffic is isolated from the rest of the network. Overall network bandwidth increases because the Accounting Dept. does not have to fight with the Engineering Dept. for access to the segment. Each segment has reduced the amount of traffic on it and the result is faster access. Each department still has complete access to the other segments but only when required.

Manages network traffic by filtering packets

Bridges listen to the network traffic and build an image of the network on each side of the bridge. This image of the network indicates the location of each node and the bridge's port that accesses it. With this information, a bridge can make a decision whether to forward the packet across the bridge if the destination address is not on the same port or it can decide to not forward the packet if the destination is on the same port.

This process of deciding whether or not to forward a packet is termed filtering packets. Network traffic is managed by deciding which packets can pass through the bridge. The bridge filters packets.

Translate from one protocol to another

The MAC layer also contains the bus arbitration method used by the network. This can be CSMA/CD as used in Ethernet or Token Passing as used in Token Ring. Bridges are aware of the Bus Arbitration and special translation bridges can be used to translate between Ethernet and Token Ring.

41c. Bridge Segment to Segment Characteristics

Bridges physically separate a network segments by managing the traffic based on the MAC address.

Bridges are store and forward devices. They receive a packet on the local segment, store it and wait for the remote segment's to be clear before forwarding the packet.

There are 2 physical types of bridges: Local and Remote Bridges.

Local Bridges are used as in the previous examples where the network is being locally (talking physical location now) segmented. The 2 segments are physically close together: same building, same floor, etc... Only 1 bridge is required.

Remote Bridges are used in pairs and where the network is remotely segmented (again talking physical locations). The 2 segments are physically far apart: different buildings, different floors, etc... 2 x Half Bridges are required: one at each segment. The Remote bridges are 1/2 of a normal bridge and may use several different communications media inbetween.

41d. Bridge Methodologies

There are 3 primary bridging methodologies used by bridges for connecting local area networks:

Transparent bridges Spanning Tree Protocol

Source Routing

Transparent Bridges were originally developed to support the connection of Ethernet networks. The spanning tree protocol was developed to improve upon transparent bridging. Source Routing Bridges are used by Token Ring. Source routing bridges require a solid understanding of Token Ring concepts and as such will be covered under the section discussing Token Ring.

Transparent Bridges

Transparent Bridges examine the MAC address of the frames to determine whether the packet is on the local Segment or on the distant Segment. Early bridges required the system administrator to manually build the routing table to tell a bridge which addresses were on which side of the bridge. Manually building a routing table is called fixed or static routing. Modern bridges are self-learning, they listen to the network frame source addresses to determine which side of the bridge the node is on and build a routing table that way.

The following network will be used as an example of a self-learning transparent bridge's routing table construction.

As frames flow on Bridge #1's local port, Bridge #1 examines the source address of each frame. Eventually after all nodes on the local port, have become active, Bridge #1 associates their address as being on the local port. Any frames with a destination address other than the nodes on the local port are forwarded to the remote port. As far as Bridge #1 is concerned, nodes on Bridge #2's local port appear as if they were on Bridge #1's remote port.

Bridge #2 builds its routing table in a similar manner to Bridge #1. Note the differences.

Advantages to Transparent Bridges Self learning: requires no manual configuration, considered plug and work. Independent of higher level protocols (TCP/IP, IPX/SPX, Netbeui, etc..)

Disadvantages of Transparent Bridges

- Can only work with 1 path between segments: Loops are not allowed. A loop would confuse the bridge as to which side of the bridge a node was really on: local or remote?

Transparent Bridges are not acceptable for use on MANs or WANs, as many paths can be taken to reach a destination. In the above example, it is simple to determine that a loop occurs but in a large corporate network with several hundred bridges, it may be next to impossible to determine. As such, Bridges are most commonly used in LAN to LAN connectivity and not in MANs or WANs.

Spanning Tree Protocol - IEEE 802.1D

The Spanning Tree Protocol was developed to address the problems of loops in Transparent Bridging. The IEEE 802.1D (Institute of Electrical and Electronic Engineers) committee formed the Spanning Tree Protocol.

The Spanning Tree Protocol (STP) converts a loop into a tree topology by disabling a bridge link. This action ensures there is a unique path from any node to every other node in a MAN or WAN. Disabled bridges are kept in stand-by mode of operation until a network failure occurs. At that time, the Spanning Tree Protocol will attempt to construct a new tree using any of the previously disabled links.

The Spanning Tree Protocol is a Bridge to Bridge communication where all bridges cooperate to form the overall bridge topology. The Spanning Tree algorithm is dynamic and periodically checks every 1 to 4 seconds to see if the bridge topology has changed.

Bridge #3 & #5 are stand-by bridges and have their links disabled. This results in only 1 path to each network segment.

Each bridge is assigned an arbitrary number to assign priority to the bridge in the internetwork. The number is concatenated with the bridge MAC address. The MAC address is used as a tie breaker mechanism if 2 bridges have the same priority. The lower the assigned number the higher the bridge priority.

During initial power-up, a Bridge Protocol Data Unit (BPDU) is flooded out each network port of the bridge. The BPDU contains the current spanning tree root, the distance to the root (measured in hops through other bridges), the bridge address information and the age of the information in the BPDU. Bridge priorities are usually manually controlled so as to configure the traffic flow over the internetwork over a preferred path.

Problems can arise where the Spanning Tree Algorithm may select a path from Los Angeles to New York City and back to San Francisco rather than the preferred route of Los Angeles to San Francisco.

41e. Reasons to use a Bridge

There are four basic reasons to use a bridge:1. Security: Stops networks from forwarding sensitive data2. Bandwidth: Reduce traffic by segmentation

3. Reliability: If 1 segment goes down, it does not take down the complete LAN

4. Translation: Translate different Data Link protocols such as Token Ring to Ethernet

41f. Bridge Addressing

Bridges work at the Data Link Layer and recognize the MAC addresses. Spanning Tree Protocol adds a Bridge Protocol Data Unit (BPDU) for Bridge to Bridge communications. Source Route Bridges and Token Ring provide special Data Link layer communication and will be discussed later.

41g. Collapsed Backbones

Collapsed Backbones take the network backbone and electronically collapse it into a high speed electronic card cage. Usually Collapsed Backbones operate at 100 Mbps. The card cage holds plug-in cards for repeaters, hubs, bridges, routers, brouters and gateways.

Software is provided to remotely configure all plug-in cards using SNMP. SNMP is a network management protocol that stands for Simple Network Management Protocol. It is a standard for intelligent network devices to communicate their configuration to administrators operating from remote workstations. The workstations can be thousands of miles away!

42. Routers

Routers are hardware and software devices. They can be cards that plug into a collapsed backbone, stand-alone devices (rack mount or desktop) or software that would run on a file server with 2 NICs.

42a. Purpose of Routers

The purpose of a router is to connect nodes across an internetwork regardless of the Physical Layer and Data Link Layer protocol used. Routers are hardware and topology independent. Routers are not aware of the type of medium or frame used (Ethernet, Token Ring, FDDI, X.25, etc...). Routers are aware of the Network Layer protocol used: Novell's IPX, Unix's IP, XNS, Apples DDP, etc..

42b. Router OSI Operating Layer

Routers operate on the OSI Model's Network Layer. The internetwork must use the same Network Layer protocol. Routers allow the transportation of the Network Layer PDU through the internetwork even though the Physical and Data Link Frame size and addressing scheme may change.

42c. Router Segment to Segment Characteristics

Routers that only know Novell IPX (Internetwork Packet Exchange) will not forward Unix's IP (Internetwork Packet) PDUs and vice versa. Routers only see the Network Layer protocol that they have been configured for. This means that a network can have multiple protocols running on it: SPX/IPX, TCP/IP, Appletalk, XNS, etc..

In the following network, Router #3 is a Novell SPX/IPX router, it only sees the Network Layer protocol IPX. This means that any TCP/IP PDUs will not pass through, the router does not recognize the PDUs and doesn't know what to do with them.

Routers #1 & #2 are TCP/IP routers, they recognize only IP protocols. This keeps SPX/IPX traffic off of "Segment 300". This is in quotations because TCP/IP has a different network numbering scheme than IPX.

Important Point: Routers allow network traffic to be isolated or segmented based on the Network Layer Protocol. This provides a functional segmentation of the network.

Routers that only can see 1 protocol are called Protocol Dependent Routers. Routers that can see many different protocols (2 or more) are called Multiprotocol Routers.

42d. Router Addressing

Routers combine the Network Number and the Node Address to make Source and Destination addresses in routing Network Layer PDUs across an network. Routers have to know the name of the segment that they are on and the segment name or number where the PDU is going to. They also have to know the Node Address: MAC Address for Novell and the IP address for TCP/IP.

For Novell's SPX/IPX (Sequential Packet eXchange/Internetwork Packet eXchange), the Network Layer PDUs address is composed of the Network Address (32 bit number) and the Host address (48 bit - MAC address).

42e. Routing Protocols

Routing Protocols are a "sub-protocol" of the Network Layer Protocol that deal specifically with routing of packets from the source to the destination across an internetwork. Examples of Routing Protocols are: RIP, IGRP and OSPF.

42f. RIP - Routing Information Protocol

RIP was one of the first routing protocols to gain widespread acceptance. It is described in RFC1058 which is an Internet standard. RFC stands for request for comment and the RFC1058 is the 1,058 RFC standard published. Commercial NOS such as Novell, Apple, Banyan Vines and 3Com, use RIP as the base routing algorithm for their respective protocol suites.

RIP is a distance vector algorithm. Routers maintain a detailed view of locally attached network segments and a partial view of the remainder of the routing table. The routers contain information on the number of hop counts to each segment. A hop is considered to be one transverse through a router. Pass through a router and the Hop count increases by 1.

The routers are updated every 30 seconds, each router sending out a RIP broadcast. This advertisement process is what enables RIP routing to be dynamic. Dynamic routers can change routing tables on the fly as the network configuration changes. By using the Hop Count information from their routing tables, routers can select the shortest path - the least number of hops to the destination.

Apple uses RTMP (routing table maintenance protocol) which adds a route status indicator: good, bad or suspect depending on the age of the route information.

Novell adds ticks to the RIP algorithm, Ticks are dynamically assigned values that represent the delay associated with a given route. Each tick is considered 1/18 of a second.

LAN segments are typically assigned a value of 1 tick, a T1 link may have a value of 5 to 6 ticks and a 56 Kbps line may have a value of 20 ticks. Larger number of ticks indicate a slower routing path.

Three commonest problems that can occur with RIP are:

1. Routing loops: the router indicates that the shortest path is back the way the packet came from.

2. Slow Route Convergence: routers have delay timers that start counting after the RIP advertising packet is broadcasted. This gives the routers time to receive and formulate a proper routing table from the other routers. If the delay timer is too short, the routing table can be implemented with incomplete data causing routing loops

3. Hop Count Exceeded: the maximum number of hop counts is 15 for RIP. A hop count of 15 is classified as unreachable which makes RIP unsuitable for large networks where hop counts of 15 and above are normal.

42g. EGRP - Exterior Gateway Routing Protocol

EGRP was created to solve many of the problems with RIP and has become the default routing protocol across the Internet. EGRP is an enhanced distance vectoring protocol, it uses up to 5 metrics (conditions) to determine the best route:

Bandwidth Hop Count (Delay) - maximum of 255

Maximum Packet size

Reliability

Traffic (Load)

These routing metrics are much more realistic indicators of the best routes compared to simple hop counts.

42h. OSPF - Open Shortest Path First

OSPF is a link state premise, this means that it has several states of routers linked together in a hierarchical routing model:

The top of the root is the Autonomous Router, it connects to other autonomous systems (the Internet). The next is the Backbone Routers, which is the highest area in the OSPF system. Border routers are attached to multiple areas and run multiple copies of the routing algorithm. Last is internal routers which run a single routing database for one area.

Basically, by dividing the network into a routing hierarchy, substantial reduction of routing update traffic and faster route convergence results on a local basis. Each level has a smaller routing table and less to update.

43. Brouters (Bridge/Routers)Brouters are protocol dependant devices. When a brouter receives a frame to be forwarded to the remote segment, it checks to see if it recognizes the Network layer protocol. If the Brouter does, it acts like a router and finds the shortest path. If it doesn't recognize the Network layer protocol, it acts like a bridge and forwards the frame to the next segment.

The key advantage to Brouters is the ability to act as both a bridge and a router. It can replace separate bridges and routers, saving money. This is, of course, provided that the Brouter can accomplish both functions satisfactorily.

44. GatewaysOne definition of a Gateway is the Hardware/Software device that is used to interconnect LANs & WANs with mainframe computers such as DECnet and IBM's SNA.

Often the router that is used to connect a LAN to the Internet will be called a gateway. It will have added capability to direct and filter higher layer protocols (layer 4 and up) to specific devices such as web servers, ftp servers and e-mail servers.

44a. Gateway's OSI Operating Layer

A Gateway operates at the Transport Layer and above. Typically translating each source layer protocol into the appropriate destination layer protocol. A mainframe gateway may translate all OSI Model layers. For example, IBM's SNA (System Network Architecture) does not readily conform to the OSI Model and requires a gateway to tranlate between the two architectures.

44b. Gateway Segment to Segment Characteristics

There can be major differences between "local" and "distance" segments. As can be seen from the above diagram, the 2 Networks appear as if they are from other planets. Mainframes are based on a central number crunching CPU with terminals connected. All information displayed on the terminals is controlled by the central CPU.

LANs consist of distributed CPUs that share data and files. This leads to a unique problem in connecting the two architectures that requires a gateway.

44c. Gateway Addressing

The gateway addressing depends on which OSI layers are translated. It could be all layers!

45. Token RingSTOP - You are now leaving Ethernet IEEE 802.3 

Please fasten your seatbelts and place your trays in the fully upright position

Token Ring is a token passing bus arbitration topology for the Physical and Data Link Layers. It is a logical ring and a physical star topology.

Token Ring uses a token passing scheme for bus arbitration. A special packet is passed around the ring called a token. When a node requires access to the ring, the node claims the token and then passes its information packet around the ring. All nodes read the destination address and if it is not addressed for them, the information packet is then passed on to the next node. When the destination node reads the packet, it marks it as read and passes it on to the next node. When the information packet

completely circulates the ring and arrives back at the source node, the source node releases the token back on to the ring.

Token Rings are not usually drawn as the above drawing indicates: a separate line between each node. They are usually represented as understood that separate paths exist between nodes and are drawn as in the figure to the right.

45a. IBM Token Ring

Token Ring was originally developed by IBM for their PC LAN networks. It started out in 1969 as the Newhall Network, named after the originator of the token ring concept. IBM's Token Ring is the basis for the IEEE 802.5 standard Token Ring. They are very similar and have minor differences which we will cover.

45b. IEEE 802.4 Token Bus

An industrial version of Token Ring is standardized under IEEE 802.4 Token Bus. It is used in manufacturing process equipment for plant operation. It is used in automobile plants for computerized assembly. It uses a Logical Ring and a Physical Bus topology.

45c. IEEE 802.5 Token Ring

IEEE 802.5 Token Ring standard is based on the IBM Token Ring network. Token Ring has been used mainly in large corporations and was considered in the past to be the only way to handle data communications in large networks (1000+) nodes.

Token Ring equipment is more expensive than Ethernet and is one of the reasons that Ethernet is more popular. The other reason is that Token Ring is much more complex bus arbitration method than CSMA/CD and few network personnel understand the full capabilities of Token Ring.

45d. IEEE 802.5 Bus Arbitration

Token Ring is a token passing bus arbitration. A token is circulated on the ring. If a node on the ring needs to access the ring (transfer information), it claims the token.

The token is a special packet, that is circulated around the ring. It is read from one node than passed to the next node until it arrives at a node that needs to access the ring (transfer information/data). When a node receives the token, the node is allowed to send out its information packet.

Example: The token is circulating the ring, Node B needs to send some data to Node G. Node B waits for the token to come by. There is only one token allowed on the ring. When it receives the token, it can then send out its information packet. Node G is the destination address.

Node C receives the packet, reads the destination address and passes it on to the next node. Node D, E & F do likewise.

When the packet arrives at node G, node G reads the destination address and reads the information. Node G marks the information packet as read and passes it on.

Note: the Source and Destination addresses remain unchanged after passing through Node G. Node B is still the Source address and Node G is still the Destination address.

The packet continues around the ring, until it reaches the source address Node B. Node B checks to make sure that the packet has been read - this indicates that Node G is actually present. The information packet is erased. Node B then releases the token onto the ring.

Information marked READ is passed through the ring back to the Source - Node B

The information packet is called the Token Frame. The token is called the Token (sometimes referred to as the free token). This can be confusing. Remember, when we talk about a frame, we are talking about data/information. When talking about a token, we are talking about bus arbitration and permission to use the bus.

45e. 4 / 16 Mbps Transfer Rate

The transfer rate for Token Ring is 4 Mbps for older systems or 16 Mbps for newer systems (1990 and newer). There are several products in development and available that will increase Token Ring's transfer rate using Switching Hubs and even faster transfer rates over existing cabling.

NOTE: 16 Mbps NIC cards will operate at both 16 and 4 Mbps speeds.

4 Mbps NIC cards will only operate at 4 Mbps.

To identify the speed of an unknown card, exam the integrated circuits on the card. There is only 1 chipset that implements IEEE 802.5's 4 Mbps standard for Token Ring. It was developed jointly by Texas Instruments and IBM. It is a 5 chip set and consists of:

TMS38051 Ring Interface Transceiver TMS38052 Ring Interface Controller

TMS38010 Communications Protocol Processor

TMS38021 Protocol Handler for 802.5 Functions

TMS38030 DMA Controller between NIC and PC Bus

4 Mbps Token Ring NICs are usually full length expansion cards.

16 Mbps NICs have typically 1 large IC with 132 pins and several small ones. They are typically 1/2 length cards. The IC number is TMS380C16 for the Texas Instrument version or TROPIC for the IBM version or DP8025 for the National version.

45f. IEEE 802.5 Topology

Token Ring is a Logical Ring / Physical Star topology. So far we've been only discussing the logical portion. Nodes on the network are physically connected via their NICs to a central concentrator or hub. The concentrator is called a MAU or MSAU both stand for MultiStation Access Unit. To avoid confusion with Ethernet MAUs, we will refer to a Token Ring hub as a MSAU (pronounced "M sow") or as a concentrator.

45g. MSAUs

A Token Ring MSAU has connections to connect to the nodes and it also has special connections called Ring In and Ring Out to connect to other MSAUs.

The Ring In connector is abbreviated RI and the Ring Out connector is abbreviated RO. The nodes (PCs) would be attached to connectors 1 to 8 for this 8 node MSAU.

The MSAU logical connection would be drawn as indicated below:

The connection from the Node to the MSAU is called the Lobe. The connection to the ring is via the Ring In and Ring Out connectors.

MSAUs are passive devices, there isn't any "intelligence" built-in. MSAUs come in 2 flavours:

Unpowered - The unpowered MSAUs receive their power through the NIC cards.

Powered. - The powered MSAUs plug in the wall outlet and have their own power supply built-in.

Token Ring connectors

The wiring between the NIC card and MSAU consists of 2 pairs of wires:

Receive Pair - This pair receives packets from its upstream neighbour Transmit Pair - This pair transmits packets to its downstream neighbour

There are 4 types of connectors used with Token Ring:

RJ11 connectors are used with older 4 Mbps systems RJ45 connectors with both 4/16Mbps systems

Hermaphroditic connectors with IBM Cat 1 cabling

The DB9 is used to connect the NIC card to the Hermaphroditic cable.

Signal Lead Hermaphroditic RJ45 RJ11 DB9

Tx+ Orange (O) 3 2 9

Rx+ Red (R) 4 3 1

Rx- Green (G) 5 4 6

Tx- Black (B) 6 5 5

UTP wiring pinouts:

Note: The receive pair (Rx) is the center pair of wires. The transmit pair (Tx) the outside pair.

MSAU Relay

When a Token Ring NIC is first turned on, it goes through a process called Ring Insertion. It checks the Lobe to see if the wiring is okay and then applies a DC voltage on the Transmit pair of wires. The DC voltage is often called phantom power.

This voltage energizes a relay in the MSAU and attaches the Lobe to the ring. If you disconnect a cable at the MSAU, the relay will de-energize and automatically disconnect the lobe from the ring. You can actually hear the relays clicking in and out.

Ring In/ Ring Out

On a MSAU are 2 connectors called Ring In (RI) and Ring Out (RO). These are used for connecting MSAUs together. Two pairs of wires are run between MSAUs to connect them together, one pair is used for the Main Ring and one is used for the Backup Ring.

The following figure indicates the Main Ring and the Backup Ring. Notice that the Backup Ring runs in parallel with the Main Ring and is not normally used. Also notice that the direction of data flow on the Backup Ring is opposite to the Main Ring.

Wrapping

If the Main Ring fails due to cable faults or MSAU problems, the Main Ring can be wrapped to the Backup Ring. Wrapping is a term that is used to indicate that the Backup Ring is being used in addition to the Main Ring.

The Backup Ring is connected to the Main Ring. The Main Ring or a portion of the Main Ring is still being used. Wrapping is only associated with the Ring In and Ring Out connectors on the MSAUs.

Main Ring wrapped to Backup ring

This can be done either of 3 ways:

Passive Hermaphroditic Style MSAUs - remove the suspected RI or RO Hermaphroditic connector. The connector will automatically short and wrap the Main Ring to the Backup Ring

Passive RJ11 & RJ45 Style MSAUs - Manually switch the suspected RI or RO connector with the available switches

Active MSAUs - They will automatically wrap if there is a problem.

Physical Star/ Logical Ring With an understanding of how an MSAU works, it is easier to see how we get a

Logical Ring for Token Ring. The Physical Star results from the Lobe cabling fanning out to the Nodes.

45h. IEEE 802.5 and the OSI Model

45i. Token Ring Cabling

There are 2 basic types of Token Ring cabling: Shielded Twisted Pair (STP) Unshielded Twisted Pair (UTP)

Shielded Twisted Pair

STP or Shielded Twisted Pair is balanced shielded twisted pair cable, 150 +/-15 ohms impedance. It is used typically with the Hermaphroditic connectors. It is referred to as IBM Type 1, 1A, 2 or 6 cabling. It is the most expensive cabling to use. The cable is expensive and the connectors are expensive.

Max Lobe Distance # Stations per ring Concentrator

4 Mbps 1000 ft/ 305 m 250 Passive

16 Mbps 550 ft/ 168 m 250 Passive

4/16 Mbps 1000 ft/ 305 m 250 Active

Unshielded Twisted Pair - Type 3

UTP or Unshielded Twisted Pair is used with phone style connectors: RJ11 or RJ45. It is 100 +/-15 ohms impedance typically 22 to 24 AWG wire. It is categorized into the following categories:

Max Lobe Distance # Stations per ring Concentrator

4 Mbps 328 ft/ 100 m 72/54 Passive (old/new)

16 Mbps 328 ft/ 100 m 250 Passive

4 Mbps 328 ft/ 100 m 54 Active

16 Mbps 328 ft/ 100 m 250 ActiveIBM Cabling System

Type 1

Two shielded, solid wire, twisted pairs, 22 AWG. Available for plenum or nonplenum interior use and underground or aerial exterior use. Use of Type 1 permits transmission at 16 Mbps and a maximum of 260 stations on the network.

Note: Plenum is heating ducts and air returns. To be qualified for plenum installation means that it must meet certain standards for releasing hazardous fumes and temperature ratings.

Type 2

Two shielded, solid wire twisted pairs, 22 AWG plus four twisted pairs of solid 26 AWG wires added between the shield and the insulating cable sheath. Type 2 supports 16 Mbps transmission.

Type 3 (RJ11 and RJ45)

Unshielded, telephone grade (22 or 24 AWG) twisted pairs, typically found inside a building. Basically equivalent to Cat 5 cable. See previous section on Unshielded Twisted Pair cabling.

Type 5

100/50 micron fibre-optic cable, used to connect distant MSAUs with fibre optic repeaters.

Type 6

Patch cables consisting of data-grade, stranded, shielded twisted pairs, 26 AWG. the distant limits are 66% of Type 1 cable.

Type 8

Under carpet cable, data-grade twisted pair cable, 26 AWG. The distance limits are 50 percent of Type 1.

Type 9

Shielded twisted pair, 26 AWG approved for plenum installations. The distance limits are 66% of Type 1 cable.

45j. Ring Insertion

When any node or host wishes to attach to the ring, it initiates the Ring Insertion process. The Ring Insertion process has 5 phases:

Phase 0 Lobe Media Check

The Lobe Media Check is performed by the NIC and it verifies the Lobe cable by looping the station transmit signal to the station receiver at the MSAU. A Lobe Media test MAC frame is issued. The relay in the MSAU is not energized at this time. A special packet is sent from the NIC to the de-energized MSAU lobe relay. The packet loops back from the MSAU and returns to the NIC. The integrity of the wiring that makes up the lobe is be checked.

The NIC applies Phantom Power (DC voltage) on the Transmit pair to activate the relay at the MSAU port. The NIC is now physically connected to the Ring.

Phase 1: Monitor Check

The ring station waits for an Active Monitor Present frame, Standby Monitor Present frame or Ring Purge MAC frame. If the ring station does not receive one of these frames before the T(attach) timer runs out, the ring station initiates Token Claiming to re-elect an Active Monitor.

Phase 2: Address Verification

The station verifies that its MAC address is unique with the Ring. It sends a Duplicate Address Test MAC frame onto the ring. The Duplicate Address Test frame has the source and destination address set to its own MAC address. If the frame returns marked read, then the station knows that there is another node with an identical address.

Phase 3: Neighbour Notification

The station learns its Nearest Active Upstream Neighbour (NAUN) address and informs its downstream neighbour of its own address through the Neighbour Notification process.

Phase 4: Request Initialization

The workstation sends the Request Initialization MAC frame to the Ring Parameter Server (RPS), which responds with an Initialize Ring Station MAC frame containing the station's parameters, such as the local ring number, ring parameter timer values, etc.. If no RPS is available, the station will insert with its default parameters.

When Phase 4 is complete the station is physically and logically attached to the ring.

45k. CAUs & LAMs

Smart concentrators or Hubs are called CAUs (pronounced cows) in Token Ring. CAU stands for Control Access Unit. It has a CPU built in and the smarts to control and determine when a Node is operating incorrectly. It can determine if the RI or RO main ring is operating properly. CAUs can make decisions on disconnecting nodes or wrapping the Main Ring to the Backup Ring. They are also able to be controlled and programmed from a remote station - SNMP compliant (Simple Network Management Protocol). Nodes can be remotely disconnected from the Ring. CAUs controls LAMs.

A CAU can control up to 4 LAMs (pronounced lambs). LAM stands for Lobe Access Module and LAMs have the Lobe connections. The CAU is connected to the LAMs by a Power Connection and a Data Connection. A LAM has 20 lobe connections. A LAM is an active concentrator.

Active Concentrators

An active concentrator is a concentrator that retimes and regenerates the data signal. It does the job of a repeater. Since it retimes and regenerates the data signal it is not used in Ring Length calculations.

45l. Ring Calculations

Maximum Ring Length

The ring length of a Token Ring network is based on the length of the cable used in concentrator-to-concentrator connections, and in the longest concentrator-to-node connection. It is based on Type 1 cabling.

Ring Speed Maximum Ring Length

4 Mbps 1200 ft/ 360 m

16 Mbps 550 ft/ 168 m

Ring Length Calculations

The following ring has Type 1 cabling, 4 passive concentrators and the lobe with the Maximum Lobe Length (MLL) indicated. The cable length for this ring is calculated by adding all the cable lengths between the passive concentrator's Ring In and Ring Out connectors together plus the Maximum Lobe Length (MLL).

Total Cable Length = MML + RI/RO cable lengthsTotal Cable Length = 40 ft + 6 ft + 75 ft + 230 ft + 80 ft = 431 ft

The passive concentrators (MSAUs) will have an effect on the ring length also. Each passive concentrators will appear as 25 ft of Type 1 cable. The Ring Length has to be adjusted for the presence of each of the MSAUs:

Ring Length = Cable Length + (number of MSAUs x 25 ft)Ring Length = 431 ft + 4 x 25 ft = 531 ft

If you check the maximum ring length parameters mentioned earlier, you will see that this ring would function within the specifications for both a 4 Mbps and a 16 Mbps Token Ring.

Mixing Cable Types and Ring Length

The Ring Length is always calculated based on Type 1 cable. All other types of cable used in the network are converted to Type 1 cable first before determining the Ring Length. The conversion factors for other cable types is indicated in the following table:

Cable Type Conversion Factor

Type 1, 1A or 2 STP 1.0 (this is the reference)

Type 6 STP 1.3

Cat 5 UTP 1.7

Cat 3 UTP 3.0For example, in the following Token Ring, there is a mixture of cable types. The first step is to convert the cable lengths to their equivalent Type 1 cable length.

Cable Type Length Conversion Factor Type 1 equivalent length

A Cat 5 UTP 80 ft 1.7 136.0 ft

B Type 6 STP 12 ft 1.3 15.6 ft

C Cat 5 UTP 65 ft 1.7 110.5 ft

D Cat 3 UTP 127 ft 3.0 381.0 ft

E Type 2 STP 185 ft 1.0 185.0 ft

Total Cable Length = 828.1 ft

Ring Length = Total Cable Length + (number of MSAUs x 25 ft)Ring Length = 828.1 ft + (4 x 25 ft) = 928.1 ft

If you check the maximum ring length parameters, you will see that this ring would function within the specifications for a 4 Mbps but not for a 16 Mbps Token Ring.